Chemical   Monographs 

UC-NRIUF 


CHEMISTRY  OF 
COMBUSTION 


J.  NEWTON  FRIEND 


D,  Van  Nostrand  Company 


CHEMICAL    MONOGRAPHS 

EDITED  BY  A.  C.  GUMMING,  O.B.E.,  D.Sc.,  F.I.C. 


The  Chemistry  of  Combustion 


CHEMICAL     MONOGRAPHS 

EDITED  BY  A.  C.  GUMMING,  O.B.E.,  D.Sc.,  F.I.C. 

THE  progress  of  Chemistry  is  so  rapid  that  it  is 
becoming  a  matter  of  ever-increasing  difficulty  to 
keep  abreast  of  the  modern  developments  of  the 
science.  The  volume  of  periodical  literature  is  so 
enormous  that  few  can  hope  to  read,  far  less 
assimilate,  all  that  is  published.  The  preparation 
of  summaries  has  therefore  become  a  necessity,  and 
has  led  to  the  publication  of  various  well-known 
journals  devoted  to  the  abstraction  of  original  papers. 
For  obvious  reasons,  however,  these  do  not  fully 
supply  the  wants  of  advanced  students  and  research 
workers,  and  it  is  now  generally  recognised  that 
monographs  on  special  subjects  are  also  needed. 

This  series  of  monographs  is  intended  primarily  for 
Advanced  and  Honours  students.  As  each  mono- 
graph is  written  by  an  author  with  special  knowledge 
of  the  subject,  and  copious  references  are  given,  it  is 
hoped  that  the  series  will  prove  useful  also  to  those 
engaged  in  research. 

The  following  volumes  are  ready  : — 

THE  ORGANOMETALLIC  COMPOUNDS  OF  ZINC  AND  MAGNESIUM. 
By  HENRY  WEEN,  M.A.,  D.Sc.,  Ph.D.,  Head  of  the 
Department  of  Pure  and  Applied  Chemistry  at  the  Muni- 
cipal Technical  Institute,  Belfast. 

THE  CHEMISTRY  OF  DYEING.  By  JOHN  KERFOOT  WOOD,  D.Sc., 
F.I.C.,  Lecturer  on  Physical  Chemistry,  College  of  Tech- 
nology, Manchester. 

THE  CHEMISTRY  OF  KUBBER.  By  B.  D.  PORRITT,  F.I.C.,  M.Sc. 
THE  FIXATION  OF  ATMOSPHERIC  NITROGEN.  Second  Edition. 

By    JOSEPH     KNOX,    D.Sc.,    Lecturer    on    Chemistry, 

University  of  Glasgow. 
THE  CHEMISTRY  OF  LINSEED  OIL.    By  J.  NEWTON  FRIEND, 

D.Sc.,  Ph.D.,  F.I.C.,  Head  of  the  Chemistry  Department, 

Municipal  Technical  School,  Birmingham. 

THE  CHEMISTRY  OF  COAL.  By  JOHN  BRAITHWAITE  KOBERTSON, 
M.A.,  B.Sc.,  A.I.C.,  Lecturer  on  Chemistry  at  the  South 
African  School  of  Mines  and  Technology,  Johannesburg. 


The  Chemistry  of 
Combustion 


BY 


J.    NEWTON    FRIEND 

D.Sc.(B'ham.),   Ph.D.(Wiirz.),   F.I.C. 

Carnegie  Gold  Medallist ;  Head  of  the  Chemistry  Department,  Municipal 
Technical  School,  Birmingham 


NEW    YORK 

D.   VAN    NOSTRAND   COMPANY 

EIGHT   WARREN    STREET 

1922 


$• 


\3> 


PREFACE 

THIS  little  book  is  the  outcome  of  a  series  of  lectures 
delivered  to  my  senior  students  in  the  Chemical 
Department  of  the  Birmingham  Municipal  Technical 
School  during  the  Session  1920  to  1921.  There 
appeared  to  be  no  small  Text-book  or  Monograph 
to  which  the  students  could  be  referred  dealing 
with  the  subject  in  its  more  modern  aspect,  and 
it  was  felt  that  the  publication  of  my  lectures 
might  serve  to  fill,  however  imperfectly,  a  very 
obvious  gap  in  our  literature. 

My  sincerest  thanks  are  due  to  .Dr  A.  Parker, 
who  has  carefully  read  through  the  proof  sheets 
and  made  many  valuable  suggestions.  I  am  glad 
also  to  take  this  opportunity  of  thanking  Miss  Annie 
R.  Russell,  B.Sc.,  of  this  School,  for  assistance  and 
advice. 

J.  NEWTON  FRIEND. 


November  1921. 


490086 


CONTENTS 


PAGE 

SECTION  I.— DEFINITIONS  .....         1 
SECTION  II. — PHLOGISTON  9 

SECTION  III.— THE  COMBUSTION  OF  SOLID  CARBON     .        12 
SECTION  IV.— FLAME         .  .  .  .  .19 

SECTION  V.  — THE  COMBUSTION   OF    GASEOUS  HYDRO- 
CARBONS AND  OTHER  GASES  .  .  .36 

SECTION  VI.  —IGNITION  TEMPERATURES  .  .  .47 

SECTION    VII.  —  THE     INFLAMMATION     OF     GASEOUS 

MIXTURES     ......       60 

SECTION  VIII. —PROPAGATION  OF  FLAME  IN  GASEOUS 

MIXTURES     ......        69 

SECTION  IX.— SURFACE  COMBUSTION      .  .  .89 

BIBLIOGRAPHY  AND  NOTES  .  '  .  .101 

INDEX  109 


vii 


THE 
CHEMISTRY  OF  COMBUSTION 

SECTION    I. 
DEFINITIONS. 

AN  old  Spanish  proverb  states  that  "Where  there 
is  no  hook  to  be  sure  there  will  hang  no  bacon." 
With  equal  truth  it  might  be  said  that  where  there 
are  no  definitions  to  be  sure  there  can  be  no  science. 
For  science  is  organised  knowledge,  and  unless  our 
terms  are  carefully  defined  our  dissertations  will 
inevitably  be  diffuse  and  obscure,  if  not  indeed 
actually  misleading. 

In  no  field  of  science  is  the  need  for  a  clear 
conception  of  the  meaning  of  certain  'terms  more 
necessary  than  in  that  now  under  discussion.  The 
knowledge  that  certain  substances  will  burn  must 
be  almost  as  ancient  as  man  himself.  Such  terms 
as  fire,  flame,  and  combustion  have  for  centuries 
been  household  expressions,  and  as  such  they  have 
been  frequently  used  to  denote  phenomena,  which 
to  the  popular  mind  may  appear  like,  but  which 
in  reality  are  widely  separated  from  each  other. 
In  a  critical  discussion,  therefore,  of  the  various 
phases  of  combustion,  it  becomes  necessary,  if  the 

A 


.2 ,    DEFINITIONS 

introduction  of  a  large  number  of  new  terms  is  to 
be  avoided,  to  give  at  the  outset  precise  meanings 
to  such  popular  terms  as  it  is  desired  to  employ. 

By  combustion  it  is  now  usual  to  imply  some 
form  of  chemical  change  accompanied  by  the  evolu- 
tion of  both  heat  and  light.  Thus,  for  example, 
the  raising  of  platinum  wire  to  redness  through  the 
agency  ,of  an  electric  current  and  the  production 
of  beautiful  optical  phenomena  by  passing  electrical 
discharges  through  Geissler  tubes  are  not  examples 
of  combustion,  for  although  both  heat  and  light 
are  emitted,  they  are  not  accompanied  by  chemical 
change.  On  the  other  hand,  combustion  takes  place 
when  phosphorus  burns  in  oxygen,  for  the  enormous 
heat  and  dazzling  light  are  accompanied  by  vigorous 
chemical  change. 

Although  in  the  large  majority  of  cases  combustion 
is  the  result  of  oxidation,  it  would  be  erroneous  to 
suppose  that  combustion  cannot  also  take  place  in 
the  entire  absence  of  oxygen  whether  free  or  com- 
bined. Thus  if  a  small  jet  of  burning  coal-gas10  is 
plunged  into  a  jar  of  chlorine,  combustion  continues, 
although  a  marked  change  takes  place  in  the 
attendant  phenomena.  The  flame  becomes  deep 
orange  in  colour,  and  volumes  of  black  soot  are 
evolved ;  the  yellowish-green  colour  of  the  chlorine 
gradually  disappears,  giving  place  to  steamy  clouds 
of  hydrochloric  acid  gas.  A  familiar  lecture  ex- 
periment is  to  plunge  yellow  phosphorus  into 
bromine  vapour  when  it  readily  burns  yielding  clouds 
of  mixed  bromides,  whilst  powdered  arsenic  dropped 
upon  liquid  bromine  immediately  inflames.  A  piece 


DEFINITIONS  $ 

of  copper  foil  burns  brilliantly  in  sulphur  vapour, 
and  a  rod  of  iron  heated  to  redness  and  pressed 
against  a  piece  of  sulphur  behaves  similarly  in  the 
vapour  thus  produced,  the  molten  ferrous  sulphide 
falling  in  scintillating  globules  to  the  ground. 

These  are,  all  of  them,  examples  of  true  combustion, 
but  of  combustion  in  the  entire  absence  of  oxygen. 
As  mentioned  above,  however,  in  the  vast  majority 
of  cases  combustion  is  accompanied  by  oxidation, 
mainly  in  consequence  of  the  fact  that  the  chemically 
active  constituent  of  the  atmosphere  is  oxygen. 
Had  our  atmosphere  consisted  of  chlorine  we  should 
still  have  been  familiar  with  combustion,  but  most 
examples  would  have  been  the  result  of  chlorination. 

One  of  the  chief  factors  which  determines  whether 
or  not  chemical  change  shall  be  accompanied  by 
combustion  is  the  magnitude  of  the  heat  evolution. 
On  applying  a  light  to  a  jet  of  hydrogen  issuing 
from  a  tube  into  the  air,  the  hydrogen  readily  burns, 
and  if  the  surrounding  air  be  replaced  by  oxygen, 
the  temperature  of  the  reaction  rises  rapidly,  an 
intense  heat  being  produced. 

The  reaction  is  thus  powerfully  exothermic,  the 
amount  of  heat  evolved  being  as  follows : — 

(H2)  +  (O)  =  (H2O)  + 58,700  calories 

at  room  temperature.  The  physical  state  of  the 
reacting  masses  is  indicated  by  the  brackets. 
Rounded  brackets  mean  that  the  substances  are 
present  in  the  gaseous  state ;  square  brackets  [  ] 
indicate  the  solid  state,  whilst  an  entire  absence 
of  brackets  indicates  the  liquid  state. 


4  DEFINITIONS 

The  above  equation  tells  us  that  gaseous  hydrogen 
and  oxygen  unite  to  form  water  vapour,  and  that 
for  every  gram-molecule  of  hydrogen  burned  or 
of  water  produced  58,700  gram-calories  of  heat 
are  evolved. 

If  the  water  vapour  is  allowed  to  condense  to 
liquid  water,  allowance  must  be  made  for  the  change 
of  state  involved  which  causes  the  evolution  of  still 
more  heat — the  latent  heat  of  steam.  This  must 
be  added  to  the  previous  amount  and  our  equation 
now^  becomes1 

(H2)  +  (O)  =  H2O  +  68,360  calories. 

The  molecule  of  water  is  not  bracketed  as  it  is  now 
assumed  to  be  in  the  liquid  state. 

But  when  hydrogen  combines  with  oxygen,  com- 
bustion does  not  always  ensue.  This  has  been  known 
for  many  years.  Hooke  in  1803  observed  that 
electrolytic  gas  confined  in  vessels  over  water  very 
slowly  combined  at  the  room  temperature,2  the 
amount  of  combination  being  appreciable  only  after 
several  months.  As  the  temperature  is  raised,  the 
rate  of  combination  steadily  increases 3  and  proceeds 
with  a  measurable  velocity  at  about  450°  C.4  There 
is  still  no  sign  of  combustion,  however,  in  the  sense 
in  which  the  term  has  been  defined  above,  so  that 
evidently  the  mere  fact  that  a  reaction  is  powerfully 
exothermic  is  not  in  itself  sufficient  guarantee  that 
it  shall  be  accompanied  by  combustion.  The  condi- 
tions obtaining  at  the  time  of  experimentation  must 
also  be  considered. 

It  is  immaterial  in  regard  to  the  quantitative  heat 


DEFINITIONS  5 

evolution  whether  one  element  combines  rapidly  with 
another,  or  slowly.  The  same  amount  of  heat  per 
gram-molecule  is  evolved  in  either  case,  provided 
the  initial  and  final  phases  are  identical  in  the  two 
sets  of  experiments.  If  the  reaction  proceeds  very 
slowly  the  heat  set  free  will  ordinarily  be  dissipated 
with  such  relative  rapidity  that  no  sensible  rise  in 
temperature  occurs,  and  the  reaction, may  even  proceed 
to  completion  without  the  phenomena  characteristic 
of  combustion  ever  appearing. 

From  the  foregoing  it  will  be  clear  that  exothermic 
chemical  reactions  may  take  place  with  all  degrees 
of  velocity,  ranging  from  that  which  proceeds  so 
slowly  as  to  be  measurable  only  after  many  months 
up  to  that  which  is  practically  instantaneous  or 
explosive.  When  a  reaction  occurs  with  a  sensible 
rise  in  temperature,  but  unaccompanied  by  light, 
it  is  frequently  termed  slow  combustion.  The 
oxidation  of  impurities  in  the  blood  by  air  drawn 
into  the  lungs  is  a  familiar  example.  In  the 
majority  of  these  cases  the  reaction  does  actually 
proceed  slowly,  and  the  term  is  unexceptionable. 
But  such  is  not  always  the  case.  Every  student  of 
chemistry  knows  that  nitric  oxide,  itself  a  colourless 
gas,  is  characterised  by  the  readiness  with  which 
it  combines  with  oxygen  to  yield  rich  brown  fumes 
of  nitrogen  peroxide.  The  reaction  is  markedly 
exothermic,  the  heat  evolution  being  as  follows  : — 

2(NO)  +  (O2)  =  (N2OJ  + 40,500  calories, 

and  if  suitable  precautions  are  taken,  a  rise  in 
temperature  can  readily  be  observed.  This  is  an 


6  DEFINITIONS 

example  of  "slow"  combustion,  as  defined  above, 
although  the  reaction  is  not  in  itself  a  "  slow  "  one. 
Bearing  in  mind,  however,  the  extended  use  of  the 
prefix  in  this  connection,  the  term  slow  combustion 
is  a  very  convenient  one  to  retain. 

Although  most  substances  require  to  be  raised 
in  temperature  before  combustion  can  ensue,  a 
number  of  reactions  are  known  in  which  the  in- 
gredients react  vigorously  with  evolution  of  light 
and  heat  practically  from  the  moment  they  are 
brought  into  contact  with  one  another.  Such  re- 
actions are  examples  of  spontaneous  combustion. 
Thus  sulphur  spontaneously  burns  in  fluorine  even 
at  -187°C.  forming  the  hexafluoride  SF6.  At 
ordinary  temperatures  both  ammonia  and  nitric  oxide 
inflame  when  brought  into  contact  with  fluorine, 
yellow  phosphorus  readily  catches  fire  in  chlorine,  and 
powdered  arsenic  instantly  inflames  on  the  surface 
of  liquid  bromine.  The  pyrophoric  metals  become 
incandescent  in  moist  air,  whilst  liquid  phosphine, 
silico-ethane,  and  many  organic  substances  imme- 
diately ignite  in  contact  with  oxygen  or  air. 

The  temperature  at  which  rapid  combustion 
becomes  independent  of  external  supplies  of  heat 
is  known  as  the  ignition  temperature. 

A  mass  of  gas  raised  to  incandescence  by  heat 
is  termed  a  flame;  this  latter  is  produced  only  in 
those  cases  of  combustion  in  which  gases  or  vapours 
are  present.  Flame,  however,  does  not  always  accom- 
pany rapid  gaseous  combustion,  a  striking  exception 
being  afforded  by  the  rapid  oxidation  of  hydrogen 
or  coal-gas  mixed  with  air  on  a  catalysing  surface, 


DEFINITIONS  7 

such  as  that  of  platinised  asbestos  or  porous  firebrick. 
Such  combustion  is  termed  flameless  or  surface  com- 
bustion, and  is  utilised  commercially  in  a  variety 
of  ways. 

A  substance  undergoing  slow  combustion  may 
exhibit  pronounced  luminosity  or  phosphorescence. 
This  is  not  an  exceptional  phenomenon,  as  was 
formerly  believed,  but  a  natural  prelude  to  rapid 
combustion,  the  appearance  of  flame  being  the  cul- 
minating point  of  a  series  of  changes,  and  coincident 
with  the  ignition  temperature.5  Many  substances 
are  now  known  to  exhibit  phosphorescence,  but  the 
temperature  of  its  appearance  is  so  high  or  the 
temperature  interval  during  which  the  phenomenon 
is  perceptible  is  frequently  so  short  as  to  be  ordinarily 
overlooked.  In  the  case  of  phosphorus  it  is  par- 
ticularly well  marked,  the  temperature  intervals, 
namely,  7°  to  60°  C.,  including  the  ordinary  range  of 
atmospheric  temperature,  so  that  the  phosphorescence 
could  hardly  be  overlooked.  Had  an  arctic  climate 
prevailed,  however,  the  phosphorescence  would  not 
have  been  so  readily  observed,  for  in  igniting  phos- 
phorus with  a  match  or  taper  the  phosphorescent 
temperature  interval  would  be  rapidly  passed,  as  is 
the  case  under  existing  conditions  with  sulphur. 
Occasionally  phosphorescence  is  referred  to  as  de- 
graded combustion.  This  is  not  a  happy  term,  how- 
ever, suggesting  as  it  does  that  combustion  is  not 
complete.  This  may  often  be  the  case,  but  it  is 
not  universally  so,  sulphur  being  a  case  in  point. 
This  element  phosphoresces  direct  to  the  dioxide, 
no  incomplete  oxidation  products  being  obtained.5 


8  DEFINITIONS 

When  a  reaction  proceeds  with  a  rise  of  temperature 
and  an  ever-increasing  velocity  until  a  high  maximum 
velocity  is  attained,  an  explosion  or  detonation 
results. 

Combustion  is  greatly  facilitated  by  fineness  of 
division.  This  is  well  illustrated  by  the  pyrophoric 
metals.  If  yellow  phosphorus  is  dissolved  in  carbon 
disulphide  and  the  solution  poured  over  some  filter 
paper,  the  solvent  rapidly  evaporates  leaving  the 
phosphorus  in  a  very  fine  state  of  subdivision,  and 
inflammation  rapidly  takes  place. 

Many  explosions  in  factories,  mines,  etc.,  have 
been  definitely  traced  to  the  presence  of  dust,  the 
activity  of  which  is  largely  a  surface  phenomenon 
akin  to  those  now  under  discussion.  The  influence 
of  dust  may  be  illustrated  in  a  harmless  manner 
by  introducing  some  very  finely  powdered  coal  or 
charcoal  into  a  gas  jar  to  the  depth  of  about  half  an 
inch,  and  blowing  oxygen  into  it  from  a  glass  tube 
reaching  to  the  bottom.  When  the  oxygen-enriched 
air  in  the  jar  is  thick  with  dust,  a  light  is  applied 
with  a  long  taper,  and  a  flame  flashes  down  the 
jar  with  explosive  violence. 


SECTION    II. 

PHLOGISTON.1 

FOR  many  centuries  prior  to  the  discovery  of  oxygen 
it  had  been  assumed  that  all  combustible  bodies 
possessed  a  combustible  principle — the  fire  matter  of 
the  Greek  philosophers.  Owing  to  its  ready  com- 
bustibility, sulphur  was  for  long  regarded  by 
European  alchemists  as  the  essential  principle  of 
combustion.  Becher  (1635-1682)  christened  this 
principle  terra  pinguis  or  "oily  earth,"  and  distin- 
guished it  from  sulphur  which,  however,  he  regarded 
as  rich  in  this  principle.  When  a  substance  burned, 
therefore,  the  oily  matter  escaped  leaving  an  incom- 
bustible rash  or  calx  (from  the  Latin  calx,  lime). 
Stahl  (1660-1'734)  developed  Becher's  views,  modified 
and  extended  them.  He  termed  the  combustible 
principle  phlogiston  from  the  Greek  <j>\oyifav,  to 
ignite.  When,  therefore,  a  metal  such  as  zinc 
burned  in  air,  phlogiston  escaped,  the  residual  ash 
of  dephlogisticated  metal  being  termed  zinc  calx. 
Thus, 

zinc  =  zinc  calx  +  phlogiston. 

The  residual  air  (nitrogen),  now  saturated  with 
phlogiston  was  termed  phlogisticated  air.  Stahl 
appears  to  have  regarded  carbon  as  almost  pure 


10  PHLOGISTON 

phlogiston,  and  other  chemists  subsequently  viewed 
hydrogen  in  the  same  light. 

So  long  as  the  adherents  of  the  phlogistic  theory 
were  content  to  regard  phlogiston  as  a  principle 
only,  and  not  as  a  material  body,  the  theory  was 
distinctly  attractive,  phlogiston  being  the  prototype 
of  what  we  now  term  the  heat  tone  or  heat  of 
reaction.  Such  a  theory,  however,  could  not  offer 
a  complete  explanation  for  many  of  the  phenomena 
known,  even  in  the  times  of  Becher  and  Stahl,  to  be 
attendant  upon  combustion,  but  the  materialisation 
of  phlogiston,  whilst  removing  some  difficulties, 
introduced  many  others.  Thus,  for  example,  the 
identification  of  phlogiston  with  hydrogen  enabled 
chemists  to  correlate  the  escape  of  this  gas  upon 
the  solution  of  metals  in  acids  with  the  combus- 
tion of  the  same  metals  in  air;  at  the  same  time 
it  introduced  the  very  serious  difficulty  that  no 
hydrogen  could  actually  be  detected  in  the  air  after 
the  combustion  of  a  metal,  and  no  answer  was 
forthcoming  as  to  whither  it  had  escaped.2 

The  two  chief  difficulties  which  ultimately  led  to 
the  overthrow  of  the  phlogistic  theory  were  the 
following : — 

1.  It  had  been  known  for  many  centuries  that 
ordinary  combustion  would  not  take  place  in  the 
absence  of  air.  Thus  the  Arab  chemist  Geber,  in 
the  eighth  century,  stated  that  the  calcination  of 
mercury  must  be  carried  out  in  open  vessels,  and 
Stahl  himself  was  aware  that  even  soot,  which  he 
regarded  as  almost  pure  phlogiston,  would  not  burn 
out  of  contact  with  air.  This  was  explained  on  the 


PHLOGISTON  11 

assumption  that  the  phlogiston  could  not  leave  a 
substance  unless  it  had  somewhere  to  go  to.  In 
other  words,  the  air  was  believed  to  act  as  an 
absorbent  for  the  phlogiston  just  as  a  sponge  sucks 
up  water  or  as  charcoal  adsorbs  colouring  matter 
from  liquids  and  bad  odours  from  gases.  This 
ingenious  explanation,  however,  takes  no  cognisance 
of  the  fact  that  the  volume  of  the  air  actually 
becomes  smaller  during  the  calcination  of  metals. 

2.  In  1630,  Jean  Rey  had,  drawn  attention  to  the 
fact  that  both  lead  and  tin  exhibit  an  increase  in 
weight  upon  calcination  in  air.  Now,  according  to 
the  phlogistic  theory, 

metal  =  calx  +  phlogiston, 

so  that  even  if  phlogiston  was  a  non-material  and 
imponderable  principle,  the  metal  could  not  weigh 
less  than  the  calx ;  whilst  if  phlogiston  was  material- 
ised into  carbon,  hydrogen,  or  any  other  substance, 
the  metal  must  weigh  proportionately  more  than  its 
calx.  This  anomaly  was  realised  by  several  chemists ; 
but  mere  recognition  of  a  difficulty  does  not  involve 
its  solution,  and  it  was  not  until  the  discovery  of 
oxygen  by  the  Birmingham  divine,  Priestley,3  that 
Lavoisier  was  able  to  offer  a  more  correct  theory 
of  calcination  and  aerial  combustion.  According  to 
him,  when  a  substance  burns  in  air  it  combines  with 
the  oxygen  to  form  an  oxide^an  explanation  that 
is  regarded  as  correct  at  the  present  time. 


SECTION  III. 

THE  COMBUSTION"  OF  SOLID  CARBON. 

IT  is  a  matter  of  common  knowledge  that  when 
carbon  is  allowed  to  burn  in  excess  of  air  or  oxygen 
the  only  product  is  carbon  dioxide  ;  whereas,  if  the 
supply  of  air  is  restricted,  carbon  monoxide  appears 
in  amounts  varying  according  to  the  conditions. 
This  is  capable  of  explanation  in  one  of  three  ways, 
namely  :  — 

1.  It  may  be  assumed  that  the  first  product  of  the 
interaction  of  carbon  and  oxygen  is  the  dioxide, 
C02,  and  that  this,  in  contact  with  excess  of  carbon, 
is  reduced  to  the  monoxide,  CO.  Thus  :  — 


(1) 

(2)  C02  +  C  =  2CO. 

^  After  the  overthrow  of  the  phlogistic  theory,  and 
during  the  major  part  of  last  century,  this  was  the 
commonly  accepted  theory.1 

2.  In  1872  Sir  Lowthian  Bell,  as  the  result  of 
long  continued  study  of  the  chemical  processes 
involved  in  iron  smelting,  concluded  that  "carbon 
monoxide  and  not  carbon  dioxide  is  the  chief,  if  not 
the  exclusive  and  immediate  action  of  the  hot  blast 
on  the  fuel  "  (coke). 

12 


COMBUSTION  OF  SOLID  CARBON  13 

According  to  this  the  presence  of  carbon  dioxide 
is  due  to  oxidation  of  the  monoxide  in  excess  of  air, 
the  gas  thus  being  a  secondary  instead  of  a  primary 
product.  Thus  :  — 


(1)  2C  +  O2  =  2CO 

(2)  2CO  +  02  =  2CO2. 

This  second  theory  received  support  from  a 
number  of  interesting  observations.  Thus  in  1887, 
C.  J.  Baker  2  observed  that  carbon,  which  has  been 
thoroughly  dried  by  exposure  to  phosphorus  pent- 
oxide,  is  allowed  to  absorb  oxygen,  dried  in  a  similar 
manner  at  12°  C.  and  is  then  heated  to  450°  C.  ;  the 
evolved  gas  consists  mainly  of  carbon  monoxide. 
Having  satisfied  himself  that,  under  the  conditions 
of  his  experiments,  carbon  dioxide  if  initially  formed 
could  not  have  been  reduced  to  the  monoxide,  Baker 
concluded  that  carbon  is  oxidised  directly  to  carbon 
monoxide  by  the  absorbed  oxygen. 

These  conclusions  were  supported  the  following 
year  by  H.  B.  Baker  3  who  showed  that  :  — 

(i.)  Thoroughly  dry  carbon  dioxide  is  not 
reduced  by  dry  carbon  even  at  bright  red 
heat. 

(ii.)  Carbon  monoxide  is  the  main  product  of  the 
combustion  of  carbon  in  dry  oxygen. 

Thus,  when  oxygen  which  had  been  thoroughly 
dried  by  prolonged  contact  with  phosphorus  pent- 
oxide  was  passed  over  highly  purified  sugar  charcoal 
at  bright  red  heat,  no  visible  combustion  occurred. 


14  COMBUSTION  OF  SOLID  CARBON 

The    gases    passing    over    possessed    the   following 
composition : — 

Oxygen      .         .         .         .58-1  per  cent. 
Carbon  monoxide        .         .       39-5         „ 
Carbon  dioxide  .         .          .          282         „ 

Bell's  theory  thus  appeared  to  be  well  substantiated. 

3.  In  1913,  however,  a  new  complexion  was  put 
upon  the  whole  problem  by  the  extensive  researches 
of  Rhead  and  Wheeler.4  These  investigators  found 
that  charcoal  which  has  been  heated  in  a  vacuum 
up  to  950°  C.  and  allowed  to  cool,  readily  absorbs  or 
occludes  appreciable  quantities  of  oxygen  at  all  lower 
temperatures.  The  amount  of  occluded  oxygen 
increases  with  fall  of  temperature,  and  remains 
surprisingly  constant  for  any  given  temperature  for 
the  particular  specimen  of  charcoal  employed. 

The  rate  of  absorption  of  oxygen  by  exhausted 
charcoal  is  exceedingly  rapid  during  the  first  fifteen 
seconds,  after  which  a  slow  absorption  continues  over 
several  hours.  This  is  well  illustrated  in  Fig.  1, 
which  depicts  the  relative  amounts  of  oxygen 
absorbed  at  various  temperatures  during  short 
intervals  of  time.  The  type  of  curve  is  seen  to  be 
the  same  for  each  temperature,  the  main  differences 
consisting  not  in  the  rates  of  fixation  of  the  oxygen, 
but  in  the  respective  quantities  of  the  gas. 

This  rather  suggests  that  the  absorption  is  a  purely 

f    physical  phenomenon.     On  the  other  hand,  it  is  found 

that  the  oxygen  clings  so  tenaciously  to  the  carbon 

as  to  defy  removal  by  the  most  complete  exhaustion 

at  any  given  temperature,  provided  that  temperature 


COMBUSTION  OF  SOLID  CARBON 


15 


remains  constant.  If  the  temperature  is  raised  by 
a  certain  amount,  a  definite  quantity  of  oxygen  is 
removed  in  vacuo  in  the  form  of  a  mixture  of  carbon 
monoxide  and  dioxide  until  the  saturation  point  of 
the  charcoal  at  this  new  temperature  is  reached,  after 


10 
TIME     IN     MINUTES. 


20 


FIG.  1. — Absorption  of  Oxygen  by  exhausted  Charcoal  (Rhead 
and  Wheeler). 

which  no  further  gas  is  removable  without  another 
rise  in  temperature. 

Thus,  if  the  temperature  were  raised  from  300°  C. 
to  350°  C.  the  amount  of  oxygen  liberated  in  vacuo 
as  oxides  of  carbon  would  correspond  to  the  fall  in 
concentration  in  the  charcoal  from  A  to  B. 

If  this  is  a  case  of  purely  physical  absorption  it 


16  COMBUSTION  OF  SOLID  CARBON 

is  surprising  that  a  reduction  in  pressure  should 
alone  be  insufficient  to  remove  either  oxygen  or  its 
oxidation  products,  the  monoxide-  and  dioxide'  of 
carbon,  at  temperatures  below  900°  C.5  Again,  one 
would  have  expected  that  admission  of  either  of 
the  oxides  of  carbon  to  exhausted  carbon  at  tempera- 
tures sufficiently  low  to  avoid  chemical  action,  for 
example,  250°  C.,  would  result  in  a  partial  absorp- 
tion of  some  of  them.  This,  however,  has  been 
demonstrated  not  to  be  the  case. 

Rhead  and  Wheeler  therefore  conclude  that  the 
first  product  of  the  combustion  of  carbon  is  a  loosely 
formed  complex  which  may  be  regarded  as  an 
unstable  compound  of  carbon  and  oxygen  of  an 
unknown  formula,  C^O^.  This  is  presumed  to  be 
formed  by  the  "  fixation "  of  such  oxygen  molecules 
as  come  into  collision  with  the  carbon.  There  are 
^  good  grounds  for  believing  that  the  carbon  molecule 
is  extremely  complex  in  structure,6  and  the  authors 
suggest  that  during  oxidation  the  oxygen  molecule 
may  actually  penetrate  into  the  carbon  molecule,  a 
rearrangement  of  the  atoms  taking  place.  However 
this  may  be,  it  is  sufficient  to  assume  that  the  oxygen 
molecule  is  temporarily  fixed.  The  repeated  fixation 
of  oxygen  molecules,  however,  causes  the  evolution 
of  a  considerable  amount  of  heat,  so  that  some  of 
the  molecules  eventually  acquire  sufficient  energy  to 
seize  hold  of  a  carbon  atom  and  depart  with  it  as 
carbon  dioxide.  Some  of  them  become  torn  apart  in 
the  process  and  leave  the  carbon  molecule  as  carbon 
monoxide. 

The  formation  and  partial   decomposition  of   the 


COMBUSTION  IN  COKE-FIRED  FURNACE     17 

intermediate  compound,  CxOy,  continues  until  the 
carbon  becomes  "saturated"  with  oxygen,  after 
which  there  is  alternate  formation  and  decomposition 
of  the  complex.  Each  oxygen  molecule  that  impinges 
on  the  carbon  liberates  so  much  energy  upon  fixation 
that  the  equivalent  amount  of  monoxide  or  dioxide 
is  liberated  by  disruption  of  a  certain  quantity  of 
CxOy  formed  from  previous  oxygen  molecules. 

This  attractive  theory,  whilst  not  definitely  proven, 
appears  to  fit  in  extremely  well  with  known  facts, 
and  is  not  at  variance  with  any  of  the  arguments 
brought  forward  in  support  of  either  of  the  two 
previous  theories. 

Composition  of  the  Complex,  CxOy. — The  quantity 
of  oxygen  absorbed  by  a  sample  of  charcoal  at  300°  C. 
in  one  experiment  amounted  to  0-16  gram  per  12 
grams  of  the  latter,  so  that  the  empirical  composi- 
tion of  the  complex  would  be  represented  by  the 
formula,  C100O. 

On  the  other  hand,  the  relative  proportions  of 
carbon  dioxide  and  monoxide  evolved  on  raising  the 
temperature  of  saturated  charcoal  is  found  to  vary 
with  the  initial  temperature,  so  that  it  would  appear 
impossible7  from  available  data  to  determine  the 
actual  values  for  x  and  y. 

Combustion  in  a  Coke-fired  Furnace. 

When  a  current  of  ordinary  air  is  injected  into  a 
furnace  on  to  a  mass  of  incandescent  carbon  (coke), 
the  complex  CXO  initially  formed  is  rapidly  decom- 
posed yielding  a  mixture  of  the  two  oxides  of  carbon. 

B 


18  COMBUSTION  OF  SOLID  CARBON 

» 
These  tend,  also   rapidly,    to   reach   a   condition   of 

equilibrium  as  represented  by  the  equation 
2CO^  C  +  CO2. 

Accordingly,  if  the  time  required  to  attain  to  this 
equilibrium  at  high  temperatures  is  less  than  that 
taken  by  the  gaseous  phase  to  traverse  the  fuel 
bed,  given  constant  pressure  and  temperature,  the 
ratio  C02/CO,  as  found  in  the  emergent  gases, 
should  remain  constant. 

If  the  coke  is  at  a  high  temperature,  namely,  not 
less  than  1200°  C.,  then  approximately  99-94  per 
cent,  of  the  gaseous  mixture  consists  of  carbon 
monoxide,  and  the  reaction  may  be  considered  as 
taking  place  according  to  the  equation 

2C  +  O2  =  2CO  + 58,000  calories, 

although,  theoretically,  no  matter  how  high  the 
temperature  attained,  a  small  but  definite  proportion 
of  carbon  dioxide  must  always  be  present  to  maintain 
equilibrium. 


SECTION  IV. 
FLAME. 

A  FLAME  has  already  been  defined  as  a  mass  of  gas 
raised  to  incandescence  by  heat.  According  to  this 
definition  it  is  possible  for  us  to  have  flame  without 
combustion,  and  this  may  be  experimentally  realised 
during  the  passage  of  electricity  through  rarefied 
gases.  Such  is  the  exception  and  not  the  rule, 
however,  and  our  concern  is  with  those  flames  that 
are  a  manifestation  of  combustion. 

Perhaps  the  most  satisfactory  method  of  studying 
flames  is  to  take  a  few  typical  examples  and  examine 
them  in  detail. 

The  Candle  Flame. 

The  candle  flame  is  a  never-failing  source  of 
interest.  The  fuel  is  solid,  but  is  gradually  liquefied 
by  the  heat  generated  when  once  combustion  has 
been  started,  and  lies  in  a  cup-shaped  hollow  at 
the  foot  of  the  wick.  By  capillary  attraction  some 
of  this  molten  fuel  is  continually  being  drawn  up 
the  wick,  and,  reaching  a  much  hotter  zone,  is  not 
only  vaporised  but  ignited.  If  the  candle  flame  is 
blown  out,  and  the  whitish  vapours  of  paraffin 
escaping  from  the  wick  are  brought  immediately 

19 


20 


FLAME 


into  contact  with  a  lighted  taper  (and  thus  not  given 
time  to  cool  and  condense),  a  flame  from  the  latter 
instantly  darts  across  the  intervening  space  to  the 
wick,  and  the  candle  relights. 

The  flame  consists  of  gases  which,  at  the  tem- 
perature prevailing,  are  lighter  than  the  surrounding 
air,  so  that  the  flame  rises.  This  is  a  fortunate 


NON    LUMINOUS 
MANTLE 


LUMINOUS  ZONE 


NON  LUMINOUS 
INNER    ZONE 


BLUE   ZONE 


FIG.  2. 

circumstance,  although  in  a  sense  it  is  purely  an 
accident.  One  result  is  that  as  the  flame  burns, 
cool  air  is  drawn  up  from  below,  as  shown  in  Fig.  2, 
and  keeps  the  outer  edge  of  the  wax  cool.  It  thus 
enables  the  molten  fuel  to  lie  in  a  cup-shaped  re- 
ceptacle of  solid  fuel  without  overflowing,  as  already 
mentioned.  If  the  wick  is  not  central,  or  if  the 
candle  is  not  cylindrical,  or  finally,  if  the  flame  is 


THE  CANDLE  FLAME 


21 


exposed  to  a  draught,  this  balance  is  disturbed  and 
may  result  in  molten  wax  overflowing  and  running 
down  the  sides  where  it  congeals  in  characteristic 
streaks.  As  a  rule,  therefore,  ornate  candles  are  not 
as  efficient  as  the  plain,  cylindrical  ones.  Examina- 
tion of  the  candle  flame  itself  reveals  a  number  of 
interesting  points.1  It  is  possible  to  distinguish  four 
parts  in  the  flame,  namely  : — 

(1)  The  inner,  non-luminous  zone  (see  Fig.  2) 
which     consists    essentially     of     vaporised 


FIG.  3. 

paraffin  wax.  It  has  not  had  a  chance  to 
burn  as  yet,  for  appreciable  amounts  of  air 
have  not  penetrated  so  far.  It  represents 
the  first  stage  in  the  gasification  of  the 
wax. 

If    a  narrow   glass  tube   is  inserted   in   the 
flame  as  shown  in  Fig.  3  with  its  lower  end 


22  FLAME 

in  the  non-luminous  zone,  whitish  gases  are 
seen  to  ascend  the  tube  and  to  escape  at 
the  top.  They  closely  resemble  the  vapours 
obtained  when  the  candle  flame  is  suddenly 
blown  out,  and  consist  of  vaporised  paraffin 
mixed  with  some  products  of  combustion. 
If  the  tube  is  warm  and  not  too  long  the 
gases,  upon  ignition,  will  continue  to  burn 
at  the  top,  affording  a  replica  of  the  candle 
flame  below. 

The  temperature  near  the  apex  of  this  zone 
approaches  1000°  C. 

(2)  Surrounding  the  inner,  non-luminous  zone  is 

a  luminous  portion  known  as  the  luminous 
mantle  or  zone.  Here  the  temperature 
ranges  from  1000°  to  1300°  C.  Chemical 
change  has  now  set  in,  to  be  completed  in 

(3)  The  non-luminous  outer  mantle,  where  the 

carbon  and  hydrogen  are  completely  oxidised 
in  excess  of  air  to  carbon  dioxide  and  water. 
This  is  the  hottest  part  of  the  flame  and 
it  is  in  this  mantle  that  the  wick,  already 
y  partially  carbonised,  oxidises  away  com- 
pletely and  thus  does  not  require  to  be 
snuffed,  as  it  would  do  if  it  went  straight 
up  into  the  main  portion  of  the  luminous 
mantle,  where  it  would  interfere  with  the 
normal  burning  of  the  candle. 

(4)  A  small  blue  zone  is  usually  distinguishable 
just  beneath  the  wick. 


CAUSES  OF  LUMINOSITY  23 


Causes  of  Luminosity. 

The  question  now  arises  as  to  what  actually  causes 
the  luminosity  of  the  zone  immediately  within  the 
non-luminous  outer  mantle  of  the  candle  flame. 

Before  attempting  to  draw  any  general  conclusion, 
let  us  examine  a  few  of  the  more  prominent  features 
of  the  luminous  and  non-luminous  portions  of  the 
flame. 

(1)  On  introducing  a  cold  surface  of  porcelain 

into  the  outer  non-lum.inous  mantle  no 
visible  change  occurs,  but  if  once  this 
mantle  is  penetrated  and  the  luminous  zone 
allowed  to  impinge  upon  the  cold  porcelain, 
the  latter  immediately  becomes  coated  with 
a  deposit  of  soot.  There  is  thus  a  distinct 
difference  between  the  two  portions  of  the 
flame,  the  luminous  portion  behaving  exactly 
as  if  it  contained  in  suspension  small 
particles  of  soot  or  carbon  at  white  heat. 
On  the  other  hand,  a  precisely  similar 
deposit  would  be  obtained  by  the  decom- 
position of  heated,  dense  hydrocarbons  under 
like  treatment.  Hence  this  experimental 
result  is  capable  of  a  double  interpretation. 

(2)  If  a  jet  of   hydrogen  issuing  from  a  glass 

tube  is  ignited,  the  flame  is  at  first  almost 
colourless,  but  as  the  tube  becomes  warm 
the  hydrogen  flame  becomes  yellow  in  conse- 
quence of  sodium  escaping  from  the  glass. 
If,  however,  the  hydrogen  be  made  to  issue 


24  FLAME 

from  a  platinum  tube  and  to  burn  in  a 
pure,  dust-free  air,  the  flame  is  invisible  and 
colourless.  It  may,  however,  be  rendered 
luminous  in  at  least  two  ways,  namely, 
either  by  the  introduction  of  solids,  or 
simply  by  the  purely  physical  act  of  in- 
.  creasing  the  pressure. 

The  increased  luminosity  consequent  upon 
introduction  of  solids  into  the  flame  is 
utilised  commercially  in  the  incandescent  gas 
mantles,  the  introduction  of  which  entirely 
revolutionised  the  methods  of  illuminating 
private  and  public  buildings. 

On  the  other  hand,  the  fact  that  mere 
increase  of  pressure  on  the  hydrogen  flame 
will  increase  its  luminosity  shows  that  solids 
are  not  essential  to  produce  this  effect. 

(3)  When  the  light  from  vapours  and  gases  is 
passed  through  a  spectroscope,  a  line 
spectrum  is  obtained.  By  increasing  the 
pressure,  however,  the  width  of  these  lines 
increases,  until  ultimately  a  continuous 
spectrum  is  obtained,  similar  to  that  result- 
ing from  an  incandescent  liquid  or  solid. 
A  candle  flame  likewise  shows  a  continuous 
spectrum,  so  that  its  illuminating  con- 
stituents must  either  be  solids,  liquids,  or 
highly  dense  vapours. 

x       Conclusion. — It   is   now   evident   that   the    early 

y     theory  of  Davy  (1815),  namely,  that  the  luminosity 

of  a  candle  flame  is  due  to  the  separation  and  raising 


THE  COAL-GAS  F^AME  25 

to  white  heat  of  solid  particles  of  carbon  by  incom- 
plete combustion,  is  not  the  only  explanation  that 
can  be  offered.  For  many  years  Davy's  theory  was 
generally  accepted,  but  during  the  latter  half  of 
last  century  opinion  inclined  towards  Frankland's 
view  (1867)  that  the  luminosity  is  due  to  the 
presence  of  dense  gases2  produced  in  the  flame 
as  the  result  of  incomplete  combustion;  and  con- 
sideration of  the  above-mentioned  observations  shows 
that  this  theory  also  is  quite  in  accordance  with 
known  facts.  It  may  well  be  that  a  complete 
explanation  may  include  both  of  these  views,  and 
it  is  by  no  means  impossible  that  the  luminescent 
substances  are  in  the  colloidal  state.20 

The  Coal-Gas  Flame. 

When  coal-gas  issuing  from  a  jet  is  ignited,  the 
resulting  flame  exhibits  the  same  three  prominent 
zones  that  characterise  the  candle  flame.  A  cold 
porcelain  basin  becomes  covered  with  soot  in  the 
luminous  zone,  where  combustion  is  partial  only,  but 
undergoes  no  apparent  change  in  the  outer  mantle. 
The  inner,  non-luminous  zone  consists  merely  of 
unburnt  gas,  and  can  be  abstracted  with  a  small  tube 
precisely  as  in  the  case  of  the  candle. 

Since  the  luminous  zone  owes  its  light-giving 
power  to  the  presence  of  solids  or  of  dense  hydro- 
carbons at  a  high  temperature  formed  through  partial 
combustion,  the  question  arises  as  to  what  would 
happen  if  fresh  air  could  be  injected  straight  into 
this  portion  of  the  flame.  It  is  to  be  expected  that 
combustion  would  by  this  means  be  greatly  acceler- 


26 


FLAME 


ated  and  the  life-period  of  any  luminous  particles 
rendered  so  short  that  the  luminosity  of  the  flame 
would  be  greatly  reduced,  if  not,  indeed,  made  to 
disappear  entirely. 


.AIR 


GAS 


FIG.  4. — A  Model  Bunsen  Burner. 

Such  in  general  is  the  case,  as  is  well  illustrated 
by  the  Bunsen  burner,  a  useful  form  of  which,  for 
demonstration  purposes,  consists  of  a  short  piece  of 
combustion  tubing,  some  6  or  8  in.  in  length,  and  a 
piece  of  narrower  tubing  bent  at  right  angles  and 
connected  with  the  gas  supply  (Fig.  4). 


THE  COAL-GAS  FLAME  27 

A  cursory  examination  of  the  flame  shows  that 
it  consists  of  two  parts  only,  namely,  an  inner  zone 
of  unburnt  gas  and  an  outer,  also  non-luminous 
mantle  in  which  vigorous  combustion  is  taking 
place. 

The  inner  portion  is  cool,  so  cool  that  a  match 
head  may  be  thrust  into  it  and  kept  there  unburned, 
whilst  the  wooden  stem  ignites  at  the  junction  of 
the  inner  and  outer  zones.  A  piece  of  paper  held 
momentarily  in  a  plane  at  right  angles  to  the  mouth 
of  the  burner  at  the  level  A  becomes  charred  in  a 
ring,  the  inner  portion  remaining  unaltered.  At  B, 
above  the  apex  of  the  inner  zone,  the  paper  is 
charred  in  a  disc.  Similar  results  are  obtained  with 
wire  gauze,  a  red  hot  ring  being  formed  at  A  and 
a  red  hot  disc  at  B. 

The  question  of  the  various  temperatures  attained 
in  a  Bunsen  flame  has  been  the  subject  of  some  little 
discussion,  and  published  figures  show  a  remarkably 
wide  variation.  The  data  shown  in  Fig.  5  are 
probably  fairly  accurate.3 

If,  whilst  the  flame  is  burning,  the  gas  supply  be 
gradually  reduced,  the  flame  becomes  smaller  and 
smaller,  begins  to  flicker  and  darts  down  the  tube, 
and  we  find  an  ordinary  gas  flame  now  burning  at 
the  end  of  the  bent  tube.  This  is  called  striking 
back.  The  mixture  of  air  and  coal-gas  passing  up 
the  tube  of  a  Bunsen  flame  in  normal  operation  is 
explosive  and  the  flame  tends  to  strike  back. 
Opposing  that  action  is  the  cooling  due  to  the  tube 
and  also  the  velocity  of  the  mixed  gases  up  the 
tube.  By  reducing  this  latter,  however,  a  point  is 


28 


FLAME 


reached  when  the  threatened  explosion  does  take 
place  and  the  flame  rushes  back — but  obviously 
only  so  far  as  the  explosive  mixture  extends, 
namely  to  the  supply  tube. 


-I550°C. 


-IOOO°C. 


300°C. 


4* 


FIG.  5.—  The  Temperature  of  the  Bunsen  Flame. 


SmiihelU  Separator*  —  By  an  ingenious  arrange- 
ment due  to  Smithells,  it  is  possible  to  catch  the 
flame  in  the  act  of  striking  back.  Two  pieces  of 


THE  COAL-GAS  FLAME 


combustion  tubing  are  taken,  about  8  and  10  in.  in 
length  respectively.  The  longer  piece  has  an  external 
diameter  of  about  0-2  in.  less  than  the  shorter,  and 
is  attached  to  the  chimney  of  a  Bunsen  burner 
(Fig.  6). 


COMPLETE 
COMBUSTION 


CO,COa,Ha.H20 


PARTIAL 
COMBUSTION 


BUNSEN 
CHIMNEY 


FIG.  6.— Smithells'  Separator. 

By  wrapping  a  little  paper  or  a  rubber  ring  round 
the  narrower  tube  the  outer  one  may  be  loosely 
attached  concentrically.  On  lighting  the  gas  at  the 
top  a  normal  Bunsen  flame  is  produced.  On  reducing 
the  pressure  of  the  gas,  the  flame  is  seen  to  flicker 
as  before  and  part  passes  down  the  outer  tube  until 
it  reaches  the  inner  one,  up  which  the  gases  are 
passing  with  a  higher  velocity  in  consequence  of  its 
narrower  diameter.  Here  the  flame  stops.  We 


30 


FLAME 


have  now  apparently  two  flames.  In  the  lower  one 
partial  combustion  takes  place,  and  a  mixture  of 
water,  hydrogen,  and  the  oxides  of  carbon  passes 
to  the  top  where  combustion  to  water  and  carbon 
dioxide  is  completed. 

If  a  piece  of  wire  gauze  is  gradually  brought  down 
on  to  a  Bunsen  flame  from  above,  it  will  be  seen  that 
the  flame  appears  to  be  pressed  down,  and  may  even 
be  extinguished  if  the  gauze  is  brought  as  low  as  the 
top  of  the  chimney.  This  is  because  the  wire  mesh- 
work  rapidly  conducts  the  heat  away  from  the  flame, 

—  --,-  -*-  -^  -  GAUZE 


FIG.  7. 

cooling  the  combustible  gases  on  their  passage 
through  the  gauze  to  a  temperature  below  their 
ignition  point.  If  the  gauze  is  held  stationary 
across  the  flame  for  a  few  seconds,  it  becomes  red 
hot,  as  we  have  already  seen,  and  presently  the  gas 
above  ignites.  The  heated  wires  do  not  now  reduce 
the  temperature  of  the  penetrating  gases. 

If,  on  the  other  hand,  the  wire  gauze  is  laid  on 
the  top  of  the  Bunsen  chimney,  and  the  gas  above 
it  ignited,  the  gauze  may  be  raised  some  distance 
carrying  the  flame  with  it.  As  the  heated  gases 
tend  to  rise  and  are  already  above  the  gauze,  the 
latter  keeps  cool  and  there  is  little  tendency  for  the 


INFLUENCE  OF  PRESSURE  31 

flame  to  travel  down.  This  fact  is  made  use  of  in 
the  household  incandescent  burners,  some  gauze 
invariably  covering  the  pipe  through  which  the  air 
and  gas  are  supplied,  in  order  to  prevent  striking 
back. 

This  is  the  principle  of  the  Davy  Safety  Lamp,  for 
use  in  mines  and  other  places  where  the  escape  of 
combustible  gases  renders  it  dangerous  to  carry 
naked  lights.  Numerous  modifications  have  been 
introduced  from  time  to  time  in  the  lamp,  but  the 
principle  remains  the  same.  An  oil  fed  wick  is 
surrounded  by  a  glass  cylinder,  the  upper  part  of 
which  is  closed  by  a  cylinder  of  wire  gauze  through 
which  fresh  air  for  combustion  passes  inwards  and 
burned  air  outwards  through  the  upper  portions. 
Should  any  marsh  gas  or  fire-damp  be  present  it 
exerts  a  peculiar  influence  on  the  flame.  Small 
quantities  cause  a  flickering,  whilst  with  larger 
quantities  the  flame  becomes  increasingly  elongated. 
The  combustion,  however,  will  not  ordinarily  pass 
from  the  inside  of  the  lamp  through  the  wire  gauze, 
so  that  the  danger  of  explosions  is  greatly  minimised. 

Influence  of  Pressure  on  Luminosity. 

In  1859  Frankland2  burned  six  candles  at 
Chamounix  and  found  a  loss  in  weight  of  94  grams 
per  candle  per  hour.  The  same  candles  were  burned 
on  the  summit  of  Mount  Blanc  and  were  found  to 
lose  9-2  grams  on  the  average.  Hence  the  difference 
in  pressure  had  not  materially  affected  the  rate  of 
combustion.  Frankland  observed,  however,  that  the 


FLAME 


inner,  non-luminous  zone  of  the  flame  on  the  mountain 
top  was  larger  than  in  the  valley  and  the  luminosity 
appreciably  less.  On  returning  to  England  he 
carried  out  a  series  of  photometric  measurements 
on  the  influence  of  pressure  on  the  luminosity  of 
a  candle  flame,  and  was  able  to  deduce  the  following 
law: — 

The  diminution  of  illuminating  power  is  directly 
proportional  to  the  diminution  of  atmospheric 
pressure. 

For  every  fall  of  1  in.  of  the  mercury  barometer, 
the  luminosity  falls  by  5-1  per  cent.  Hence,  taking 
that  at  London  as  the  standard,  namely  100,  at 
Miinchen  it  would  be  91,  and  at  Mexico  61-5. 

Frankland  next  showed  that  the  law  was  trust- 
worthy even  up  to  a  pressure  of  three  atmospheres, 
but  at  still  higher  pressures  the  luminosity  rapidly 
increased,  an  observation  attributable  to  less  complete 
combustion.  His  results  were  as  follows  : — 


Pressure  in 
Atmospheres. 

Observed 

Luminosity. 

Calculated 
Luminosity. 

1 
2 
3 

100 
263-7 
406 

100 
253 

406 

4 

959 

559 

Even    an   alcohol   flame    becomes    very   luminous 
under  a  pressure  of  four  atmospheres. 


INFLUENCE  OF  TEMPERATURE     33 


Influence  of  Temperature  on  Luminosity. 

As  a  general  rule,  increase  of  temperature  causes 
a  simultaneous  increase  in  the  luminosity.  If  a 
flame  is  cooled  by  bringing  near  to  it  a  block  of 
cold  metal,  a  distinct  reduction  in  luminosity  may 
be  made  apparent.  On  the  other  hand,  by  warming 
coal-gas  as  it  passes  through  the  chimney  of  a  Bunsen 
burner,  the  normally  non-luminous  flame  may  be 
made  decidedly  luminous. 


Causes  of  the  Decreased  Luminosity  of 
the  Bunsen  Flame. 

The  question  now  arises  as  to  why  the  Bunsen 
flame  should  be  non-luminous.  This  arises  from 
several  causes.5 

First,  the  injection  of  oxygen  into  the  heart  of 
the  flame  causes  the  rapid,  complete  combustion  of 
the  coal-gas  as  already  indicated.  This,  however, 
is  only  part  of  the  explanation. 

Second,  the  dilution  of  the  flame  with  atmospheric 
nitrogen  tends  to  prevent  the  formation  of  inter- 
mediate luminous  products  by  increasing  the  tempera- 
ture necessary  to  bring  about  the  necessary  partial 
decomposition  of  the  hydrocarbons. 

Third,  the  air  introduced  into  the  flame  is  cold, 
and  thus  tends  to  reduce  the  effective  heating 
influence  brought  about  by  the  oxidation  consequent 
upon  the  admission  of  atmospheric  oxygen. 


FLAME 


Reciprocal  Combustion. 

It  must  not  be  overlooked  that  a  flame  such  as 
we  have  been  considering  is  simply  a  boundary 
between  the  combustible  gases  where  chemical  com- 
bination is  proceeding  rapidly.  It  is  usual  to  regard 


FIG.  8.— Coal-Gas  burning  in  Air,  and  Air  burning  in  Coal-Gas. 

coal-gas  as  combustible,  but  this  is  simply  a  matter 
of  convenience,  for  coal-gas  is  not  combustible  in 
so  far  as  nitrogen  is  concerned,  and  oxygen  is 
combustible  where  coal-gas  is  concerned.  That  a 

o 

flame  of  coal-gas  burning  in  air  is  simply  the  inverse 
of  air  burning  in  coal-gas  may  be  very  effectively 
shown  by  the  experiment  illustrated  in  Fig.  8. 

The  coal-gas  supply  is  turned  on  full  and  ignited 


RECIPROCAL  COMBUSTION  35 

at  the  top  of  the  lamp  glass,  which  may  be  protected 
by  wire  gauze  at  A  to  prevent  cracking.  The  gas 
is  turned  down  slightly  to  create  a  draught  up  B 
and  a  lighted  taper  is  carefully  passed  up.  A  flame 
appears  at  C  as  soon  as  the  gas  is  reached — air 
burning  in  coal-gas.6 


SECTION  V. 

THE  COMBUSTION  OF  GASEOUS  HYDROCARBONS 
AND  OTHER  GASES. 

THE  problems  connected  with  the  combustion  of 
gaseous  hydrocarbons  have  been  the  subject  of 
considerable  controversy.  During  the  greater  part 

^  of  last  century  the  theory  of  preferential  combustion 
was  widely  accepted,  according  to  which  there  is 
competition  between  the  different  constituents  of 
the  burning  gases  for  the  oxygen  of  the  air.  If, 
therefore,  a  hydrocarbon  gas  undergoes  partial  com- 
bustion in  a  limited  supply  of  air,  the  "  most 
favoured"  element  will  tend  to  burn  first,  leaving 
the  remainder  to  oxidise  as  best  it  may.  In  this 
way  the  luminosity  of  a  hydrocarbon  flame,  such 
as  that  of  ethylene  or  acetylene,  received  explana- 
tion.1 The  gas  is  first  decomposed  into  hydrogen 

y  and  carbon  at  the  high  temperature  of  the  flame. 
The  hydrogen  being  under  these  conditions  the 
favoured  element  rapidly  burns  to  steam,  whilst 
the  less  favoured  carbon  remains  suspended  in  the 
flame  in  a  white  hot  condition,  thereby  rendering  it 
luminous.  Ultimately  the  carbon  itself  burns,  and 
the  process  of  combustion  is  thus  completed. 


COMBUSTION  OF  HYDROCARBONS          37 

On  the  other  hand  Dalton  2  had  found,  in  the  early 
years  of  last  century,  that  marsh  gas,  exploded  with 
its  own  volume  of  oxygen,  yields  equal  volumes  of 
steam,  carbon  monoxide,  and  hydrogen.  Thus  : 


Similarly,   ethylene  yields  carbon   monoxide   and 
hydrogen  : 


C2H4 


These  results  received  support  from  the  researches 
of  Kersten3  in  1861,  and,  in  later  years,  of  many 
other  investigators.  They  are  of  particular  interest 
in  that  they  prove  hydrogen  to  be  no  longer  the 
"  favoured  "  element  under  explosive  conditions. 

Kersten4  sought  to  explain  the  phenomena  on  the 
assumption  that  when  the  hydrocarbon  has  been 
decomposed  by  the  heat  of  the  flame  into  hydrogen 
and  carbon,  the  latter  is  preferentially  oxidised  to 
carbon  monoxide,  after  which  any  excess  of  oxygen 
distributes  itself  between  this  gas  and  the  hydrogen. 

Assuming  the  theory  of  preferential  combustion 
to  be  true,  the  question  at  once  arises  as  to  what 
factors  determine  whether  or  not  a  given  element 
shall  be  the  more  "  favoured."  Clearly  the  chemical 
nature  of  the  element  cannot  be  the  sole  deciding 
factor,  otherwise  it  is  not  clear  why  hydrogen  should 
in  certain  cases  be  more  favoured  than  carbon,  and 
in  other  cases  less  so. 

Although  the  theory  of  preferential  combustion 
has  not  been  definitely  disproved,  a  more  satisfactory 


38  COMBUSTION  OF  HYDROCARBONS 

explanation  of  the  foregoing  and  of  other  phenomena 
of  combustion  is  afforded  by  what  may  be  termed 
the  Association  theory,  according  to  which  the 
oxygen  of  the  air  first  combines  with  the  hydro- 
carbons forming  more  or  less  unstable  hydroxylated 
products  which  ultimately,  in  a  sufficiency  of  air  or 
oxygen,  decompose  to  carbon  dioxide  and  water. 

These  conclusions  have  been  arrived  at  by  Bone 
and  his  collaborators  mainly  as  the  result  of  an 
extensive  series  of  researches  on  the  slow  combustion 
of  methane  at  temperatures  ranging  from  300°  C.  to 
510°  C.,  and  receive  ample  support  from  the  behaviour 
of  mixtures  of  oxygen  and  other  hydrocarbon  gases 
under  analogous  conditions.  The  initial  experiments  5 
were  conducted  with  various  mixtures  of  gases 
contained  in  sealed  bulbs  of  boro-silicate  glass  at 
temperatures  between  300°  C.  and  400°  C.,  but  as 
the  volume  of  gases  capable  of  being  dealt  with  in 
this  manner  was  limited,  namely  about  70  c.c.,  and 
as,  moreover,  such  a  method  was  not  adapted  for  the 
detection  and  isolation  of  transient  intermediate 
products,  later  experiments 6  were  conducted  with 
the  aid  of  a  different  type  of  apparatus.  This  con- 
sisted of  a  combustion  tube  filled  with  fragments  of 
ignited  porous  porcelain  through  which  some  1200  c.c. 
of  the  reacting  gases  were  continuously  circulated  byx 
means  of  a  Sprengel  pump  which  worked  auto- 
matically. The  experiments  were  protracted,  in 
some  cases  extending  over  several  weeks,  during 
which  times  the  following  facts  were  definitely 
established : — 

1.  None  of  the  reactions  a  to   f  indicated   below 


COMBUSTION  OF  HYDROCARBONS          39 

takes  place  to  any  appreciable  extent  at  temperatures 
of  400°  C.  downwards. 

(a)      C+H9O 


(6) 
(c) 
O/)  2CO  +  O9  -  2CO9 

0) 


Also,  the  following  pairs  of  gases  were  found  to 
have  no  appreciable  mutual  action  at  or  below  400°  C,, 
namely,  CH4+C02,  CH4  +  H90,  and  CO  +  H8. 

This  greatly  simplifies  the  study  for,  when  once 
the  methane  has  begun  to  oxidise,  an  indefinite 
number  of  secondary  reactions  might  be  expected  to 
take  place  if  the  foregoing  reactions  were  capable 
of  proceeding  with  an  appreciable  velocity. 

2.  Between  300°  and  400°  C.  methane  is  oxidised 
with  comparative  rapidity  by  oxygen  gas.  When 
insufficient  oxygen  is  present  to  completely  oxidise 
the  methane,  the  final  products  are  water,  carbon 
monoxide,  and  carbon  dioxide.  Neithe'r  free  hydrogen 
nor  free  carbon  is  produced  in  detectable  quantities. 
Since,  if  once  formed,  it  would  be  impossible  for 
them  to  be  oxidised  away  in  accordance  with 
schemes  6,  c,  or  /,  it  follows  that  their  detection 
and  isolation  would  be  an  easy  matter,  and  hence 
it  may  be  postulated  that  under  normal  conditions 
of  slow  combustion  the  methane  is  not  first  dissoci- 
ated into  carbon  and  hydrogen.  It  seems  equally 
clear,  moreover,  that  the  carbon  monoxide  and  water 
formed  are  two  of  the  primary  disintegration  products 


40          COMBUSTION  OF  HYDROCARBONS 

of  the  partial  oxidation  of  the  methane  molecule  at 
these  temperatures  as  these  are  too  low  for  reaction 
/  to  take  place. 

3.  A     large     proportion     of     carbon     dioxide    is 
frequently     formed,     sometimes     almost     equal     in 
volume    to   the   carbon   monoxide   produced.     Since 
the  conditions  preclude  all  possibility  of  its  forma- 
tion by  oxidation  of   the    monoxide  either  directly 
with   oxygen   or    through    interaction    with    steam 
(reactions  d  and   e  above),  it   would   appear   to   be 
a   disintegration    product    of    some    more    complex 
oxygenated  molecule. 

4.  Formaldehyde,  H.CHO,  is  formed   during   the 
slow   oxidation   of   methane  at  450°  to  500°  C.   and 
can  be  detected  as  a  transient  intermediate  product. 
Since  it  is  not  produced  when  mixtures  of  moist  carbon 
monoxide  and  hydrogen  are  continuously  circulated 
for  two  days  over  a  hot  surface  of  porous  porcelain  at 
460°  to  480°  C.,6' 7  it  seems  reasonable  to  suppose  that 
it  is  one  of  the  products  of  the  slow  oxidation  of 
methane,  and   is  not  produced  by  minor  secondary 
reactions.     When   heated,  in   the   absence  of  air  or 
oxygen,  it   readily   decomposes   into   hydrogen   and 
carbon  monoxide  : 

H.CHO  =  H2  +  CO, 

but  there  is  no  evidence  that  the  reaction  is  revers- 
ible, although  under  the  influence  of  a  silent  electric 
discharge  formaldehyde  may  be  obtained  from 
hydrogen  and  carbon  monoxide.8  In  the  presence 
of  air  it  yields  carbon  dioxide  and  water. 

Piecing    all    this    evidence    together,    Bone    and 


COMBUSTION  OF  METHANE 


41 


Wheeler  in  1903  came  to  the  conclusion  that  the 
slow  combustion  of  methane  takes  place  in  several 
stages  involving  the  formation  and  subsequent 
decomposition  of  formaldehyde,  with  the  final  pro- 
duction of  carbon  dioxide  and  water  if  sufficient 
oxygen  is  present.  Armstrong,9  however,  suggested 
that  the  real  initial  product  is  not  formaldehyde 
but  methyl  alcohol,  which  rapidly  decomposes  to 
formaldehyde  and  steam.  This  view  is  accepted 
by  Bone.10  These  changes  may  be  represented 
schematically  as  follows  : — 


H 

H 

H 

H.C.  H  - 

>  H.C.  OH  -+ 

H.C.  OH 

H 

Methane. 

H 

Methyl 
alcohol. 

OH 

Hypothetical 
dihydroxy. 
methane. 

H 


+  H2O 

Formaldehyde 
+  Steam. 


OH 


OH 


— >  H  .  C  :  O  — >  HO  .  C :  O 


Formic 
acid. 


Carbonic 
acid. 


CO2  +  H2O 


The  methyl  alcohol,  which,  on  account  of  its  easy 
oxidation  cannot  be  experimentally  detected,  is 
assumed  to  undergo  hydroxylation  to  hypothetical 
dihydroxymethane.  This  instantly  decomposes  to 
formaldehyde,  which,  with  its  next  hydroxylation 
product,  namely  formic  acid,  can  easily  be  detected 
amongst  the  products  of  the  slow  oxidation  of 
methane.  The  final  stage  of  the  oxidation  is  reached 
with .  the  hydroxylation  of  formic  acid  to  carbonic 


42    COMBUSTION  OF  HYDROCARBONS 

acid,    which     immediately    undergoes     fission     into 
carbon  dioxide  and  water. 

With  ethane,  C2H6,  the  mechanism  of  slow  com- 
bustion is  believed  to  proceed  as  follows  u  : — 


CH  CH 


CHQ  CH 


OH 


CHo  CHQ .  CHO 


Lq  V-/J.J.O  V^AJlo  V/**£ 

> 


_  CH(OH)2  H20 


Ethane.  Ethyl  [Dihydroxy  Acetaldehyde 

alcohol.  ethane.]  +  Steam. 

H.CHO 
—  >          +          —  >HCOOH  —  >CO(OH)2 

CO  +  H2O  Formic  Carbonic 

Formaldehyde, 
carbon  monoxide,  Y 

C02  +  H20 

The  presence  of  ethyl  alcohol  as  the  primary 
product  of  oxidation  of  ethane  has  not  been  deter- 
mined experimentally,  for  alcohol  is,  under  these 
conditions  oxidised  far  more  rapidly  than  ethane 
itself.  Ethyl  alcohol  has,  however,  been  detected 
among  the  products  of  the  interaction  of  ethane  and 
ozone  at  100°  C.,  and  it  seems  highly  probable,  there- 
fore, that  ethyl  alcohol  is  really  the  primary  product 
of  oxidation  as  indicated  above.12 

Hydrogen,  methane,  and  ethylene  are  sometimes  13 
found  amongst  the  products  of  oxidation,  without, 
however,  any  carbon  being  liberated.  Their  appear- 
ance is  due  to  the  purely  thermal  decomposition  of 
ethane,  formaldehyde,  and  acetaldehyde.14  Thus  : 


H.CHO  -  H0  +  CO 
CH3.CHO  =  CH4 


COMBUSTION  OF  CARBON  MONOXIDE       43 

In  the  case  of  ethylene,15  the  reactions  suggested 
are  as  follows  : — 


CH2  CH2  CHOH 

->  ||  ->     ||  -->    2H.CHO 

CH2  CH(OH)  CHOH  Formaldehyde. 

Ethylene.  Vinyl  Dihydroxy 

alcohol.  ethylene 

(hypothetical). 

-->  H  .  COOH  —>  CO(OH)2 

Formic  Carbonic 

acid.  acid. 


C02+H20 

Vinyl   alcohol   could   not,   of    course,    be    experi- 
mentally detected  among  the  products. 
Acetylene  appears  to  react  as  follows  l6  :  — 

CH  C(OH)  C(OH)  H.CHO 

CH  CH  C(OH)  CO 

Acetylene.  Formaldehyde 

+  Carbon 
monoxide. 

->  H.COOH  -^  CO(OH)2'—  >  CO2  +  H2O 

Formic  Carbonic 

acid.  acid. 

It  seems  reasonable  to  conclude  that  during  rapid 
combustion  of  the  hydrocarbon  gases,  closely  similar 
reactions  obtain. 


Combustion  of  Carbon  Monoxide. 

Lavoisier  demonstrated  the  general  principle  that 
the  increase  in  weight  of  the  products  of  combustion 


44          COMBUSTION  OF  HYDROCARBONS 

in  air  is  equal  to  the  weight  of  oxygen  consumed. 
From  this  date  until  1880,  chemists  regarded  the 
combustion  of  carbon  monoxide  as  a  simple  straight- 
forward chemical  change,  represented  by  the 
equation 

2CO  +  O2  =  2CO2. 

In  his  address  to  the  Chemical  Section  of  the 
British  Association  in  1880,  however,  Dixon17 
pointed  out  that  dry  carbon  monoxide  and  dry 
oxygen  do  not  unite.  A  trace  of  water  must  be 
present.  Neither  dry  carbon  dioxide,  nitrogen,  nor 
cyanogen  was  found  to  have  any  effect,  but  such 
gases  as  hydrogen,  hydrogen  sulphide,  ether  vapour, 
ethylene,  ammonia,  etc.,  act  in  a  similar  manner  to 
water  vapour.  In  other  words,  all  substances  that 
will  form  steam  under  the  conditions  of  the  experi- 
ment are  capable  of  determining  the  combustion. 

Dixon,  therefore,  suggested  that  "  in  the  ordinary 
combustion  of  carbonic  oxide,"  the  steam  present  acts 
the  part  of  a  "  carrier  of  oxygen  "  by  undergoing 
reductions  and  successive  re-formations  :  —  • 

(1)  CO  +  H20  _  CO,  +  H2 

(2)  2 


Traube  18  disputed  the  generalisation.     He  pointed 
out  that  the  reaction 


is  reversible,  and  at  the  temperature  of  the  electric 
spark   proceeds   from   right   to   left.     He   therefore 


COMBUSTION  OF  CARBON  MONOXIDE       45 

argued  that  Dixon's  hypothesis  must  be  incorrect.19 
Having  observed  that  hydrogen  peroxide  is  obtained 
when  moist  carbon  monoxide  is  burned,  Traube 
concluded  that  the  reactions  were  as  follows  :— 

CO2  +  H2O2 


It  has  also  been  suggested 20  that  percarbonic  acid, 
H2C2O6,  is  formed  as  an  intermediate  product,  and 
this  is  supported  by  the  fact  that  a  flame  of  carbon 
monoxide  impinging  on  water  containing  a  little 
potassium  hydroxide  and  cobalt  chloride  gives  a 
precipitate  identical  with  that  obtained  with  the 
same  mixture  on  addition  of  potassium  percarbonate. 
But  this  test  alone  is  not  conclusive  since  potassium 
hydrogen  carbonate  gives  the  same  precipitate  with 
hydrogen  peroxide  in  the  presence  of  cobalt  chloride.21 

Bone,24  in  discussing  the  influence  of  hydrogen  upon 
the  rate  of  explosion  of  mixtures  of  carbon  monoxide 
and  oxygen,  suggests  that  carbon  monoxide  is  in- 
capable of  combining  directly  with  molecular  oxygen. 
The  hydrogen  serves  to  induce  the  formation  of  either 
nascent  oxygen  or  ''activated"  steam  molecules, 
both  of  which  can  oxidise  the  monoxide.  It  is 
assumed  that  oxygen  molecules  unite  in  the  flame 
each  with  four  atoms  of  hydrogen  to  yield  the  un- 
stable dihydrol  complex 

H\  /H 

>o=o/ 

H/  \H 

which  instantly  dissociates  into  a  mixture  of  free 
hydrogen  molecules,  nascent  oxygen,  and  nascent  or 


46          COMBUSTION  OF  HYDROCARBONS 

"activated"  steam.  The  carbon  monoxide  is  then 
oxidised  as  follows : 

CO  +  :  O  =  CO2 
CO  +  :OH2  =  CO2  +  H2 

Bone  does  not  apply  this  to  a  direct  consideration  of 
the  influence  of  steam  upon  the  oxidation  of  carbon 
monoxide,  but  since  steam  dissociates  at  the  tempera- 
ture of  flames  into  hydrogen  and  oxygen  it  is  easy  to 
see  how  the  above  reactions  could  take  place  and  the 
oxidation  of  carbon  monoxide  be  thereby  facilitated. 

The  Combustion  of  Cyanogen. 

Cyanogen  burns   in   a    Smithells'  separator   with 
characteristic  beauty.22     The  inner  flame  is  red,  the 
cyanogen  burning  to  carbon  monoxide.     Thus  : 
C2N2  +  02  =  2CO  +  N2. 

The  outer  flame  is  blue,  the  carbon  monoxide  now 
undergoing  further  oxidation  to  the  dioxide. 

The  Combustion  of  Hydrogen. 

It  is  a  remarkable  fact  that  although  hydrogen 
and  oxygen  unite  with  explosive  violence  when  a 
spark  is  passed  into  the  mixed  gases,  yet  the  pure 
gases  dried  over  phosphorus  pentoxide  do  not 
explode,  even  on  heating  to  redness.23  Even  at  the 
melting-point  of  silver  (960°  C.)  no  combination  takes 
place.  This  is  a  typical  example  of  the  curious  fact 
that,  as  the  methods  of  preparing  pure  substances 
become  increasingly  refined,  the  number  of  reactions 
known  to  occur  between  the  impure  reagents,  but 
not  between  them  or  only  with  excessive  slowness 
when  in  a  state  of  high  purity,  is  steadily  increasing. 


SECTION   VI. 
IGNITION  TEMPERATURES. 

THE  ignition  temperature  has  already  been  defined 
(see  p.  6)  as  that  temperature  at  which  rapid  com- 
bustion becomes  independent  of  external  supplies 
of  heat. 

The  temperature  of  ignition  is  the  temperature  at 
which  the  heat  evolved  by  the  reaction  just  equals 
and  therefore  counterbalances  the  loss  of  heat 
consequent  upon  radiation,  etc. 

A  clear  idea l  may  be  obtained  by  imagining  a 
combustible  mixture  of  gases  to  issue  from  an  orifice 
into  an  inert  atmosphere.2  If  the  orifice  is  sur- 
rounded by  a  ring  of  platinum  wire  which  is  being 
raised  in  temperature  by  passage  of  an  electric 
current,  a  flame  will  gradually  make  its  appearance. 
If,  as  soon  as  this  is  observed,  the  heating  of  the 
wire  by  the  electric  current  be  discontinued,  the 
flame  will  disappear.  It  is  not  self-supporting,  but 
depends  upon  the  accessory  supply  of  heat  from  the 
electrically  heated  wire.  If  now  we  raise  the  ring 
to  a  still  higher  temperature  than  before,  a  brighter 
flame  is  obtained  in  consequence  of  the  increased 
rate  of  chemical  action ;  and  finally  a  temperature  is 
reached  at  which  the  flame  will  continue  to  burn 
without  the  aid  of  any  heat  from  the  wire,  so  that 
the  current  may  be  cut  off.  This  is  the  temperature 
of  ignition. 

47 


48  IGNITION  TEMPERATURES 

Attention  has  already  been  directed  to  the 
phenomena  of  spontaneous  combustion.  When,  for 
example,  liquid  phosphoretted  hydrogen,  P9H4,  or 
zinc  ethyl,  Zn(C2H5)2,  is  allowed  to  come  into  contact 
with  air,  it  spontaneously  bursts  into  flame.  The 
ignition  temperatures  of  these  bodies  lies  below  the 
usual  temperature  of  the  atmosphere.  The  ignition 
temperature  of  pure  gaseous  phosphoretted  hydrogen, 
PH3,  lies  below  100°C.,  and  a  sample  of  the  gas 
prepared  by  the  action  of  alcoholic  potash  upon 
phosphorus  if  allowed  to  escape  from  a  jet,  may  be 
ignited  by  causing  it  to  impinge  upon  a  test-tube  of 
boiling  water.  The  vapour  of  carbon  disulphide 
ignites  at  a  slightly  higher  temperature,  namely,  at 
about  120°  C.  If,  therefore,  a  hot  glass  rod  is  intro- 
duced into  a  beaker  containing  a  few  drops  of  carbon 
disulphide,  the  vapour  immediately  inflames. 

Hydrogen  ignites  in  air  more  readily  than  coal- 
gas,  and  this  is  readily  shown  by  allowing  the  latter 
to  impinge  upon  a  warmed  bundle  of  platinised 
asbestos,  when  the  latter  becomes  red  hot,  but  does 
not  usually  effect  the  ignition  of  the  gas.13  If,  how- 
ever, the  coal-gas  be  replaced  by  hydrogen,  the  latter 
immediately  inflames. 

The  earliest  systematic  attempts  to  determine 
the  ignition  temperatures  of  various  gases  were 
those  of  Davy,3  who  observed  that  hydrogen  could 
be  inflamed  at  the  "lowest  visible  heat  of  iron," 
that  is  approximately  500°  C.  Ethylene  and  carbon 
monoxide  inflamed  at  red  heat  (probably  c.  700°  C.), 
whilst  methane  required  contact  with  an  iron  rod 
in  brilliant  combustion.  Modern  methods  of  deter- 


IGNITION  TEMPERATURES  49 

mining  ignition  temperatures  may  be  divided  into 
five  main  groups,  namely  : — 

I.  The  combustible  mixture  of  gases  is  contained 
in  glass  bulbs  and  raised  in  temperature  by  immersing 
in  a  heated  bath.4 

Under  these  conditions  the  gases  are  fired  under 
pressures  usually  somewhat  greater  than  atmospheric, 
and  in  contact  with   a   relatively   large   superficial 
area  of  glass  which  may  act  catalytically  upon  the 
reaction.     This  method  has  also  been   adopted    for 
slow   combustion   experiments,  but,  in   consequence 
of  the  catalytic  action  just  referred  to,  different  in- 
vestigators have  frequently  obtained  widely  different/ 
results.     Thus,  for   example,    with  electrolytic   gas,    / 
Meyer  and  Raum4  made  the  following  observations  : —     /  ^ 

Temperature. 

100°  C  .  .  No  water  detectable  after  218  days. 

300°  .  .  Water  detected  in  65  days. 

.350°  .  .  Water  detected  in  5  days. 

448°  .  .  Perceptible  action,  but  very  slow. 

On  the  other  hand,  Bone  and  Wheeler4  have  «- 
succeeded  in  keeping  electrolytic  gas  ^in  glass  bulbs 
at  400°  C.  for  seven  days  without  observing  any 
combination  of  the  gases,  although  in  some  of  their 
experiments  the  presence  of  water  could  be  detected. 
It  would  thus  appear  that  400°  C.  is  the  border-line 
for  the  slow  combustion  of  hydrogen  and  oxygen 
under  these  conditions  within  finite  time. 

II.  A  second  method  consists  in  passing  a  stream     v 
of  the  combustible  mixture  of  gases  through  a  tube 
which  is  gradually  heated  up  until  the  gases  ignite.5 
This  method  has  the  advantage  of  yielding  results 

at  the  normal  pressure  of  the  atmosphere,  but  is  still 

D 


50 


IGNITION  TEMPERATURES 


liable  to  be  influenced  by  the  catalytic  action  of  the 
walls  of  the  tube.  Further,  the  gases  are  not  at 
rest,  but  in  motion. 

III.  A  particularly  valuable  method  is  that  devised 
by  Dixon6  in  1903  and  shown  diagrammatically  in 
Fig.  9. 


f 

v 

PORCELAIN 
TUBE 

/ 

ELECTRICALLY 

'1 

/> 

HEATED 

'/ 

V 

1 

fl 

% 

'J 

A 

I      c 

/ 

7         xx 

^A 

V 

/. 

\  T 

* 

'/ 

'/ 

7. 

/ 

yt 

/ 

7, 

i 

I 

V 

/ 

/• 

KS- 

r*   .  "5  >  . 

FIG.  9. — Dixon's  Apparatus  for  determining  Ignition 
Temperatures  of  Gases. 

The  principle  of  this  method  consists  in  heating 
the  combustible  gases  separately  to  such  a  tempera- 
ture that  they  immediately  inflame  upon  coming 
into  contact.  The  combustible  gas  was  admitted  to 
the  apparatus  at  A  and  the  atmosphere  of  air  or 


IGNITION  TEMPERATURES 


51 


oxygen  at  B.  During  passage  up  the  tubes  the 
gases  attained  the  temperature  registered  by  the 
thermocouple  in  T  and  began  to  mix  at  C,  from 
which  orifice  the  combustible  gas  issued.  The 
porcelain  tube,  D,  4-5  cms.  in  diameter,  surrounded 
by  platinum  wire  and  asbestos  packing,  was  electri- 
cally heated,  the  temperature  being  allowed  to  rise 
at  the  rate  of  5°  C.  per  minute,  until  the  issuing  gas 
at  C.  inflamed  in  contact  with  the  air. 

This  method,  like  the  previous  one,  gives  results 
at  atmospheric  pressure,  but  possesses  the  further 
advantages  that  the  mixed  gases  are  only  momentarily 
in  contact  with  a  possible  catalyst,  namely,  the  small 
orifice  at  C, 

Experiment  showed  that,  on  working  with  different 
furnace  tubes,  a  constant  ignition-point  was  obtained 
when  both  the  diameter  of  the  outer  tube  and  the 
rate  of  passage  of  the  combustible  gas  through  the 
orifice  were  made  to  exceed  a  certain  minimum 
value. 

The  following  results  were  obtained.: — 


Ignition  Temperature,  °C. 

Oxygen. 

Air. 

Hydrogen 

580  to  590 

580  to  590 

Carbon  monoxide  (moist) 
Methane 

637 

556 

658 
700 

644 
650 

,   658 
,   750 

Ethane  . 

520 

630 

520 

,   630 

Propane 

490 

570 

Ethylene 

500 

519 

542 

,    547 

Acetylene 

416 

440 

406 

,    440 

Cyanogen 
Hydrogen  sul 

phid( 

i 

803 
220 

818 
235 

850 
346 

,   862 
,   379 

Ammonia 

700 

860 

52  IGNITION  TEMPERATURES 

It  will  be  observed  that  in  the  majority  of  cases 
the  ignition  temperature  in  air  is  practically  identical 
with  that  in  oxygen. 

IV.  Adiabatic  compression.7  As  is  well  known, 
when  gases  are  compressed,  heat  is  evolved,  and  if 
the  rate  of  compression  is  sufficiently  rapid  to  prevent 
the  undue  escape  of  heat,  the  gases  will  rise  in 
temperature.  By  suitably  arranging  the  apparatus 
the  gases  may  be  fired,  and  if  the  volume  of  the 
gases  at  the  moment  of  firing  can  be  determined, 
their  temperature — the  ignition  temperature  under 
compression — admits  of  calculation  from  the  well- 
known  equation 


T  representing  the  absolute  temperature,  V  the 
volume,  and  y  the  mean  ratio  of  the  specific  heats 
of  the  gaseous  mixture  at  constant  pressure  and 
volume.  The  essential  features  of  the  apparatus 
used  by  Dixon  and  Crofts7  are  shown  in  outline 
in  Fig.  10. 

The  body  of  the  apparatus  consisted  of  a  steel 
cylinder  56  cms.  in  length  and^  11  cms.  in  diameter, 
with  a  cylindrical  cavity  3-02  cms.  in  diameter 
through  its  axis.  The  lower  end  of  this  cavity  was 
opened  out  and  threaded  to  enable  a  plate  to  be 
inserted  to  close  the  bottom  of  the  cylinder  and  to 
be  kept  in  place  by  a  powerful  screw.  The  combust- 
ible mixture  of  gases  was  introduced  into  the 
apparatus  through  a  tube  A,  and  the  upper  end  of 
the  cylinder  was  closed  with  a  plug  attached  to  a 
piston,  bearing  a  head,  H,  of  great  strength,  upon 


IGNITION  TEMPERATURES 


53 


which  an  iron  block,  weighing  76  kg.  (2J  cwts.),  was 
allowed  to  fall  from  a  height  of  about  1-5  metres. 


STEEL 
WASHERS 


FIG.  10.— Dixon  and  Crofts'  Apparatus  for  determining  Ignition 
Temperatures  by  Compression. 

This    sufficed    to    drive    the   plug   well    down    the 
cylinder  and  so  to  compress  the  gases.     The  extent 


54 


IGNITION  TEMPERATURES 


of  compression  was  regulated  by  a  series  of  steel 
washers  which  could  be  arranged  to  catch  the  piston 
head  at  any  desired  position  during  its  descent.8 
The  initial  and  final  volumes  being  thus  known,  the 
temperature  after  compression  was  calculated  by 
means  of  tl\e  equation  given  above,  y  being  taken 
as  1-4.  The  temperature  of  ignition  was  found  by 
"  trial  and  error,"  the  compression  being  increased  or 
decreased  in  successive  trials  until  ignition  just  took 
place. 

The  following  results  were  obtained  : — 


Relative  Volumes  of 
the  Gases. 

Ignition 
Temperature,  °C. 

2H2  +  02 

2H2  +  2O9 
2H2+8O2 
2H2+16O2 
2H2+32O2 

526 
511 
478 
472 
never  exploded 

These  results  are  not,  of  course,  strictly  comparable 
with  those  obtained  by  the  previous  method,  owing 
to  the  high  pressures  developed,  these  being  of  the 
order  of  30  atmospheres.  The  temperatures  obtained 
are  appreciably  lower  than  that  found  for  the  ignition 
of  hydrogen  in  oxygen  by  Method  III. 

V.  The  Soap  Bubble  Method.  This  exceedingly 
ingenious  device  adopted  by  M'David 9  in  1917 
consists  in  blowing  soap  bubbles  with  the  combust- 
ible gaseous  mixture  and  allowing  them  to  impinge 
upon  an  electrically  heated  wire,  the  temperature 
of  which  is  raised  until  the  gases  ignite.  The 


FLASH-POINT 


55 


results  obtained  by  this  method  are  high  and  are 
regarded  as  not  very  reliable.10 

In  the  following  table  are  given  the  results  obtained 
by  M'David,  and  these  where  possible  are  compared 
with  those  obtained  by  Dixon  and  Coward  by 
Method  III. 


Gaseous  Mixture. 

M'David. 

Dixon  and  Coward. 

Hydrogen-air    . 

747 

580  to  590 

Carbon  monoxide-air 

931 

637    „   658 

Ethvlene-air 

1000 

500   „  519 

Coal-gas-air 

878 

... 

Benzene-air 

1062 

... 

Ether-air  . 

1033 

Petrol-air  . 

995 

... 

It  will  be  seen  that  not  only  does  a  wide  difference 
exist  between  the  different  sets  of  results,  but  the 
temperatures  do  not  follow  the  same  relative  order. 
Thus,  M'David  finds  the  ethylene  to  have  a  higher 
ignition  temperature  than  either  hydrogen  or  carbon 
monoxide,  whilst  Dixon  and  Coward  found  it  to  be 
lower. 

In  the  table  on  pp.  56  and  57  are  given  the  more 
important  results  obtained  by  the  various  methods 
described  for  the  ignition  temperatures  of  gases. 

The  temperature  at  which  the  vapour  of  a  liquid 
forms  an  inflammable  mixture  with  air  is  frequently 
termed  its  flash-point,  and  its  accurate  determination 
is  often  a  matter  of  considerable  legal  importance, 
more  particularly  in  the  case  of  low  flash-point 
paraffin  oils  sold  for  domestic  illuminating  purposes. 


56 


IGNITION  TEMPERATURES 


Gases. 

Ignition 
Temperature, 
°C. 

Method. 

Authority  and  Date. 

Hydrogen-oxygen  . 

518  to  606 

I 

Krause  and  Meyer  (1891). 

5*9 

I 

Emich(1900). 

650  to  730 

II 

Meyer  and  Kreyer  (1892). 

620  „  680 

II 

Meyer  and  Munch  (1893). 

550 

II 

Mallard  and  Le  Chatelier 

(1880). 

653  to  710 

II 

Bodenstein(1899). 

674 

II 

Mitscherlich  (1893). 

840 

II 

Gautier  and  Helier  (1896). 

845 

II 

Helier  (1897). 

580  to  590 

III 

Dixon  and  Coward  (1909). 

2H2  +  02  . 
2H2  +  2O, 

540 
514 

IV 
IV 

Falk  (1906). 

99                    99 

2H2  +  4O2 

530 

IV 

2H2  +  O2    . 

526 

IV 

Dixon  and  Crofts  (1914). 

2H2  +  2O2 

511 

IV 

99                                            99 

2H2  +  8O2 

478 

IV 

99                                            99 

Hydrogen-air  . 

580  to  590 

III 

Dixon  and  Coward  (1909). 

747 

V 

M'David(1917). 

Methane-oxygen     . 

606  to  650 

I 

Meyer  and  Freyer  (1893). 

650  „  730 

II 

99                                                     99 

656  „  678 

II 

Meyer  and  Munch  (1893). 

556  „  700 

III 

Dixon  and  Coward  (1909). 

Methane  air    . 

650  „  750 

III 

99                                                99 

Ethane-oxygen 

530  „  606 

I 

Meyer  and  Freyer  (1893). 

606  „  650 

11 

99                                                99 

605  „  622 

Meyer  and  Munch  (1893). 

520  „  630 

iii 

Dixon  and  Coward  (1909). 

Ethane-air 

520  „  630 

in 

99                                                 99 

Propane-oxygen 

545  „  548 
490  ,,  570 

ii 
in 

Meyer  and  Munch  (1893). 
Dixon  and  Coward  (1909). 

Ethylene-oxygen 

530  „  606 

i 

Meyer  and  Freyer  (1893). 

606  „  650 

ii 

99                                                     99 

577  ,,  590 

ii 

Meyer  and  Munch  (1893). 

500  „  519 

in 

Dixon  and  Coward  (1909). 

IGNITION  TEMPERATURES 


57 


Gases. 

Ignition 
Temperature, 

Method. 

Authority  and  Date. 

Ethylene-air    . 

542  to  547 
1000 

Ill 
V 

Dixon  and  Coward  (1909). 
M'David  (1917). 

Acetylene-oxygen   . 

509  to  515 

416  „  440 

II 
III 

Meyer  and  Munch  (1893). 
Dixon  and  Coward  (1909). 

Acetylene-air  . 

406  ,,  440 

III 

»»                      »» 

Propylene-oxygen  . 

497  „  511 

II 

Meyer  and  Miinch  (1893). 

Isobutane-oxygen    . 

545  „  550 

II 

»»                      »» 

I  s  obuty  len  e-oxy  ge  n 

537  „  548 

II 

»»                      »» 

Coal-gas-oxygen 

647  „  649 

II 

>»                      >» 

Coal-gas-air 

878 

V 

M'David  (1917). 

Benzene-air     . 

1062 

V 

»»            »» 

Ether-air 

1033 

V 

9)                           »» 

Cyanogen-oxygen   . 

803  to  818 

III 

Dixon  and  Coward  (1909). 

Cyanogen-air  . 

850  „  862 

III 

»»                      »» 

Carbon     monoxide- 
oxygen 

650  „  730 
650  „  730 
637  „  658 

I 
II 
III 

Meyer  and  Freyer  (1893). 

•»»                                                5» 

Dixon  and  Coward  (1909). 

Carbon     monoxide- 
air 

644  „  658 
931 

III 

V 

M'David  (1917). 

Hydrogen  sulphide- 
oxygen 

250  to  270 
315  „  320 
220  „  235 

I 
II 
III 

Meyer  and  Freyer  (1893). 
Dixon'and  Coward  (1909). 

Hydrogen  sulphide- 
air 

346  „  379 

III 

„ 

Ammonia-oxygen    . 

700  „  860 

III 

>»                      »» 

Hydrogen-chlorine  . 

240  „  270 
430  „  440 

I 
II 

Meyer  and  Freyer  (1893). 
»»                      »» 

58 


IGNITION  TEMPERATURES 


An  elaborate  and  carefully  standardised  apparatus 
is  then  necessary. 

A  determination  of  the  flash-point  of  an  oil  is 
frequently  desirable  as  a  check  upon  its  purity. 
Thus,  for  example,  linseed  oil  has  normally  a  flash- 
point of  250°  C.,  but  if  adulterated  with  rosin  oil, 
of  usual  flash-point  155°  to  160°C.,  the  presence 
of  this  latter  oil  is  readily  detected  in  this  way. 
This  is  a  great  advantage,  because  although  rosin  oil 
is  denser  than  linseed,  the  density  of  the.  mixture 
could  easily  be  "  doctored "  by  addition  of  some 
other  lighter  ingredient  which  would  render  gravity 
determinations  useless. 

A  convenient  rough  method  of  determining  the 
flash-point  consists  in  heating  the  oil  in  the  inner 
pot  of  a  glue  pot,  the  outer  one  serving  as  an  air- 
bath.  A  thermometer  is  inserted  in  the  oil  and  a 
small  gas  jet  made  from  a  mouth  blow- pipe  is 
brought  close  to  but  not  actually  touching  the 
surface  of,  the  oil.  The  temperature  is  taken  at 
which  a  small  blue  flame  is  seen  to  flash  for  the  first 
time  across  the  oil.  This  is  the  flash-point.  In  the 
following  table  are  given  the  approximate  flash- 
points of  several  well-known  liquids : — 


Liquid. 

Flash-point,  °C. 

Linseed  oil  . 

250 

Rosin  oil 

155  to  160 

Paraffin  illuminating  o 

Is 

38 

50 

Turpentine  . 

S5 

40 

Rosin  spirit. 

35 

40 

Naphtha       . 

16 

21 

Methylated  spirit 

14 

16 

IGNITION  TEMPERATURES 


59 


The  ignition  temperatures  of  a  few  substances  that 
are  solid  at  the  ordinary  temperature  have  been 
determined,  and  a  few  of  the  more  interesting  data 
are  given  in  the  accompanying  table,  together  with 
the  melting-  and  boiling-points  of  the  substances 
concerned  n : — 


Substance. 

Meltinp-point, 
•C. 

Ignition 
Temperature,  °C. 

Carbon  —  Diamond 

c.  3500 

800  to  850 

Graphite 

n 

690 

Amorphous 

»» 

345 

Phosphorus,  Yellow 
Red  . 

44-5 

c.  60 
255  to  260 

Sulphur  in  air  12 
,,        in  oxygen12 

119-2 

255 
257  to  264 

SECTION   VII. 
THE  INFLAMMATION  OF  GASEOUS  MIXTURES. 

UPON  introducing  a  source  of  heat  into  a  combustible 
mixture  of  gases,  two  conditions  must  be  satisfied  in 
order  to  ensure  propagation  of  the  flame  throughout 
the  mixture. 

(a)  The  initial  source  of  heat  must  be  of  sufficient 
volume,  intensity,  and  duration  to  raise  the  adjacent 
layer  of  combustible  gases  to  at  least  their  ignition 
temperature. 

(6)  The  heat  resulting  from  the  combustion  of 
this  layer  must  be  sufficient  in  turn  to  raise  the 
next  adjacent  layer  to  its  temperature  of  ignition, 
and  so  on. 

Any  excess  of  one  of  the  constituent  gases  over 
that  required  for  complete  combustion  will  serve  as 
a  diluent,  and  if  the  excess  is  very  great  the  heat 
absorbed  in  raising  the  temperature  of  the  mixture 
may  be  so  large  as  to  prevent  the  attainment  of  the 
ignition  temperature.  The  largest  quantity  of  the 
diluent  which  may  be  present  and  yet  allow  a  flame 
to  be  propagated  from  layer  to  layer  throughout  the 
mixture  without  the  continued  presence  of  the 
original  source  of  heat,  is  a  measure  of  the  limit  of 
inflammation.  The  majority  of  the  researches  on 

60 


COMBUSTION  OF  AMMONIA  VAPOUR       61 

this  branch  of  our  subject  have  been  carried  out  with 
gases  that  are  combustible  in  air  or  oxygen,  and  it  is 
to  these  that  our  attention  will  now  be  directed. 

An  interesting  example  is  afforded  by  ammonia 
vapour  which  readily  burns  in  oxygen  but  not  in 
ordinary  air.  If  a  taper  is  applied  to  ammonia 
vapour  as  it  issues  from  a  jet,  the  characteristic  livid 


OXYGEN 


FIG.  11. — Combustion  of  Ammonia  Vapour  in  Air  enriched 
with  Oxygen. 

flame  of  the  gas  is  seen  burning  side  by  side  with 
the  flame  of  the  taper.  But  combustion  at  once 
ceases  when  the  taper  is  withdrawn.  If,  however, 
the  jet  is  surrounded  by  oxygen  or  air  enriched  with 
this  gas,  the  ammonia  may  be  made  to  burn  con- 
tinuously even  after  the  removal  of  the  taper.  This 
is  conveniently  shown  by  means  of  the  apparatus 
figured  above. 


62     INFLAMMATION  OF  GASEOUS  MIXTURES 

Concentrated  ammonia  solution  is  gently  warmed 
in  the  flask  and  the  escaping  vapour  is  surrounded  by 
air  enriched  with  oxygen  which  enters  the  circum- 
scribing cylinder  and  is  distributed  round  the  central 
tube  by  the  glass  wool  C.  If,  before  passage  of 
oxygen,  a  lighted  taper  is  applied,  the  ammonia 
vapour  is  seen  to  burn  alongside  of  the  flame  of  the 
taper,  but  immediately  the  taper  is  removed  com- 
bustion ceases.  On  adding  increasing  quantities  of 
oxygen,  the  ammonia  flame  grows  stronger  until 
eventually  the  taper  may  be  removed  and  the 
ammonia  continues  to  burn  without  requiring  any 
external  source  of  heat.  The  minimum  quantity  of 
oxygen  required  to  maintain  this  condition  is  termed 
the  lower  oxygen  limit  of  inflammation. 

By  the  lower  limit  of  inflammation  is  understood 
"the  smallest  quantity  of  any  combustible  gas  which, 
when  mixed  with  air  or  oxygen,  will  admit  of  this 
self-propagation  of  flame.  But  there  is  also  a 
higher  limit  of  inflammation,  for  clearly  if  the 
combustible  gas  is  in  excess,  the  excess  will  function 
as  a  diluent,  and  the  higher  limit  thus  becomes  the 
lower  oxygen  or  atmosphere  limit. 

^  Some  of  the  earliest  work  on  the  subject  was  that 
of  Davy1  in  1816,  whose  experiments  indicated  a 
lower  limit  of  inflammation  of  fire-damp  in  air  as 
one  part  in  16  or  17,  that  is  between  6-3  and  6-7  per 
cent.  Modern  researches  have  been  carried  out  in 
three  different  ways,  namely : — 

1.  The  gases  are  ignited  by  passing  an  electric 
spark  between  terminals  placed  at  the  centre  of 
a  large  glass  globe  containing  the  combustible 


LIMITS  OF  INFLAMMATION  63 

mixture.  This  was  the  method  adopted  by  Burgess 
and  Wheeler2  in  1911,  their  apparatus  being  shown 
in  Fig.  12. 

The  globe  had  a  capacity  of  approximately  2  litres 
and  was  fitted  with  platinum  electrodes  which 
entered  the  globe  through  ground  stoppers.  The 
method  of  experiment  consisted  of  "  trial  and  error," 
the  proportion  of  combustible  gas  being  successively 
reduced  or  increased  until  two  mixtures  were  obtained, 


FIG.  12.— Apparatus  used  by  Burgess  and  Wheeler  (191 1). 

one  of  which  just  enabled  the  flame  to  be  propagated 
whilst  the  other  did  not.  The  authors3  found  that 
the  lower  limit  mixture  could  be  distinguished  with 
certainty  from  that  containing  a  slightly  insufficient 
proportion  of  combustible  gas,  because  the  first 
sparking  produced  in  the  former  case  inflammation 
of  all  the  gas  in  the  globe,  and  on  further  sparking 
no  further  sign  of  combustion  was  manifest.  In  the 
latter  case,  on  the  other  hand,  although  the  flame 
might  appear  to  travel  pretty  well  through  the  globe 
with  the  first  spark,  a  "  cap "  would  appear  upon 


64     INFLAMMATION  OF  GASEOUS  MIXTURES 


sparking  a  second  time,  showing  that  the  whole  of 
the  combustible  gas  had  not  disappeared. 

2.  The  gases  may  be  electrically  ignited  in  a 
horizontal  tube  closed  at  both  ends.  For  this 
purpose  a  tube  about  6  cms.  in  diameter  is  con- 
venient, the  flame  steadily  creeping  along  the  upper 
walls  in  some  such  manner  as  that  indicated  in 
Fig.  13,  as  the  lower  limit  is  approached.4 


FIG.  13. 

3.  A  vertical  tube  may  be  employed,  closed  at  both 
ends  and  ignited  electrically  either  at  the  top  or  at 
the  bottom. 


Method. 

Lower  Limit. 
Methane  per  cent. 

Higher  Limit. 
Methane  per  cent. 

1.  Globe  . 

5'6 

14-8 

2.  Horizontal  tube  . 

5  '4  flame   travels 

14'3 

only  along  top 

of  tube. 

5  '6     methane    all 

combusted. 

3.  Vertical  tube  :— 

(a)  Top  ignition 

6-0 

13-4 

(ft)  Bottom  ignition  . 

not<  5-4 

not  >  14-8 

As  is  to  be  anticipated,  these  methods  do  not  all 

give  quite  the   same   result.     Owing   to   convection 

currents,  a  low  value  for  the  lower  limit  and  a  high 

value  for  the  higher  limit  are  obtained   by  bottom 

gnition  in  a  vertical  tube,  whilst  top  ignition  yields 


LIMITS  OF  INFLAMMATION 


65 


a  high  value  for  the  lower  limit  and  a  low  value  for 
the  higher  limit.  These  differences  are  well  illus- 
trated by  the  results  obtained  by  Burgess  and 
Wheeler4  for  mixtures  of  methane  and  air. 

The  lower  limit  mixture  depends  upon  a  variety 
of  factors,  the  more  important  of  which  are  as 
follows : — 

(a)  The  calorific  power  of  the  gas,  which  may  be 

designated  as  C. 

(b)  The  relative  volume  and  specific  heat  of  the 

diluent. 

(c)  The  ignition  temperature  of  the  combustible 


(d)  The  pressure.  Increase  of  pressure  raises 
the  higher  and  lower  limits,5  as  is  evident 
from  the  following  data  for  methane  in 


air: — 


Pressure, 

Lower  Limit. 

Higher  Limit. 

cm.  Mercury. 

Methane  per  cent. 

Methane  per  cent. 

76 

6'0 

13-0 

125 

6-05 

13-15 

290 

6-2 

13-6 

465 

6-4 

14-05 

(e)  The  temperature.  Rise  of  temperature  tends 
to  reduce  the  lower  limit  value,  as  theoreti- 
cal considerations  would  lead  us  to  expect. 
This  is  evident  from  the  data  given  below, 
which  refer  to  methane  in  air.6 


Temperature,  °C.    . 
Lower  Limit.  ) 

Methane  per  cent.    / 


20 
5'80 


175 

5-25 


237 
4-75 


312 
4-30 


555        690 
3*40       3-00 
E 


66     INFLAMMATION  OF  GASEOUS  MIXTURES 


(/)  In  the  case  of  the  inflammation  of  gaseous 
mixtures  in  horizontal  or  vertical  tubes,  the 
diameter  of  the  tubes  exerts  an  important 
influence  if  less  than  about  5  cms.  This  is 
well  illustrated  by  the  data  obtained  for 
acetone7  in  air. 


Lower  Limit. 

Higher  Limit. 

Per  cent.  Acetone. 

Per  cent.  Acetone. 

Diameter 

of  Tube. 

Hori- 
zontal. 

Down- 
ward. 

Upward. 

Hori- 

zontal. 

Down- 
ward. 

Upward. 

cm. 

2-5 

2-40 

2-75 

2-30 

6-7 

6-5 

7-5 

5-0 

2'25 

2'40 

2-20 

9'3 

8-3 

9-5 

10-0 

2-20 

2-35 

2-15 

9-5 

8-5 

9-7 

It  is  of  interest  to  inquire  whether  or  not  the 
lower  limit  concentration  of  a  combustible  gas  in  air 
can  be  calculated,  making  due  allowance  for  the 
above  factors.8  Assuming  d  and  e  to  represent  the 
average  pressure  and  temperature  of  the  atmosphere, 
and  either  /,  the  diameter  of  the  tubes,  to  remain 
constant,  or  the  values  for  the  lower  limits  in 
the  globe  experiments,  only,  to  be  considered,  it 
is  evident  that  factor  a,  the  calorific  power  of 
the  combustible  gas  is  the  most  important  of  the 
remaining  factors  in  the  majority  of  cases.  Writing 
L  as  the  proportion  of  combustible  gas  in  a  lower 
limit  mixture,  it  is  to  be  anticipated  that  L  should 
be  some  function  of  1/C.  If  strict  proportionality 
obtains,  which  is  very  possible  for  analogous  gases 
such  as  those  of  the  saturated  hydrocarbon  series, 
then  L  oc  1/C  or  L  =  K/C.  Taking  methane  as  the 


LIMITS  OF  INFLAMMATION 


67 


standard,  for  which  L  =  5-6  and  C  =  189-1,  we  arrive 
at  the  value 

K  =  LC  =  1059. 

Using  this  value  for  K,  L  may  be  calculated  for  the 
other  saturated  hydrocarbon  gases  from  their  calorific 
values  determined  by  Thomsen.  This  is  done  in  the 
following  table 8 :— 


Gas. 

Calorific 
Value. 

L  determined 
by  the  Globe 
Method. 

L  calculated. 

Methane 

189-1 

5-60 

standard 

Ethane  . 

336-6 

3-10 

3-15 

Propane 

484-2 

2-17 

2-19 

w-Butane 

631-7 

1-65 

1-68 

n-Pentane 

779-2 

1-37 

1-36 

Isopentane 

779-2 

1-32 

1-36 

The  agreement  is  surprisingly  close ;  but  when  this 
method  of  calculation  is  applied  to  other  combustible 
gases,  such  as  hydrogen9  (L  =  10-0),  using  the  above 
value  for  K,  serious  discrepancies  arise.  But  this  is 
to  be  anticipated,  for  there  is  no  reason  why  L  should 
be  directly  proportional  to  C — an  assumption  upon 
which  the  calculated  value  for  K  rests. 

A  few  experiments  have  been  carried  out  with  a 
view  to  determining  the  composition  of  the  atmo- 
sphere that  extinguishes  flames.  It  appears  that  the 
extinction  of  a  flame  is  not  determined  only  by  the 
proportion  of  the  "inert"  gas  and  oxygen  in  the 
surrounding  atmosphere.  The  nature  of  the  inert 
gas  also  plays  an  important  part  in  determining  the 
lower  oxygen  limit.  Thus,  fbr  example,  carbon 
dioxide  is  found  to  exert  a  more  powerful  extinctive 
effect  than  nitrogen. 


68     INFLAMMATION  OF  GASEOUS  MIXTURES 


The    composition    of    the    extinctive    atmosphere 
produced  by  a  candle  flame  is  as  follows 10 : — 


Oxygen 
Nitrogen 
Carbon  dioxide 


Per  cent. 

15  to  16 

80   „    81 

3 


This  closely  corresponds  to  the  average  composition 
of  the  air  expired  by  human  beings,  and  may  be 
breathed  by  most  people  without  producing  any 
distinctly  noticeable  ill-effect. 

Other  results  of  interest  are 11 : — 


Combustible  Substance. 

Residual  Air 
contains. 

Per  cent. 

Candle  burned  until  extinguished    -j 

4  to    6  CO2 
13  ,,  15  oxygen 

Alcohol  burning  on  cotton-wool  to  \ 
extinction                                       / 

6-5  CO2 
11  oxygen 

Wood-charcoal  glowing  to  extinc-  \ 
tion                                                 / 

8CO2 
9  oxygen 

Sulphur  burning  to  extinction 

13'5  oxygen 

Decrease  of  pressure  raises  the  lower  oxygen  limit, 
as  is  evident  from  the  following  data 12 : — 


Combustible  Substance. 

Extinction 
percentage  of 
Oxygen. 

Total  Pressure 
of  Gases. 

Ethyl  alcohol  burning  from  \ 
asbestos  wick                     / 

15-1 
19-0 

mm. 
736-7 
129 

Candle        .        .                 •  { 

16-1 
19-9 

736-7 
91 

SECTION  VIII. 
PROPAGATION  OF  FLAME  IN  GASEOUS  MIXTURES. 

A  THOROUGH  knowledge  of  the  rate  of  propagation 
of  flame  in  mixtures  of  air  and  various  inflammable 
gases,  especially  hydrocarbons,  is  eminently  desirable 
in  view  of  the  ever-present  danger  of  serious  fires 
and  explosions  in  coal-mines. 

When  an  explosive  mixture  of  gases  contained  in 
a  horizontal  tube  closed  at  one  end  is  ignited  at  the 
open  end,  it  is  observed l  that : — 
/^?f?  i 

1.  A  flame  travels  a  certain  distance  along  the 

tube  with  a  uniform  and  relatively  slow 
velocity.  In  the  case  of  acetylene  and  air 
this  distance  is  small;  with  methane  and 
air  it  is  relatively  long. 

2.  Vibrations    are    gradually     initiated     which 

become  increasingly  intense,  the  flame 
moving  backwards  and  forwards  with  oscil- 
lations of  ever-increasing  amplitude. 

3.  The   flame   either  dies  out  or  the  remainder 

of  the  gas  in  the  tube  detonates,  initiating 
what  is  termed  the  explosion  wave. 


70  PROPAGATION  OF  FLAME 

It  is  thus  evident  that  a  flame  may  be  propagated 
throughout  a  gaseous  mixture  in  two  ways,  namely  : — 

A.  With  a  relatively  slow  motion,  characterised 

by  its  uniformity,  and 

B.  With  a  rapid  explosion  wave,  usually  accom- 

panied by  detonation. 

It  is  proposed  to  deal  briefly  with  each  of  these  in 
this  section. 

A. —  Uniform  Slow  Movement  in  the  Propagation  of 
Flame  in  Gaseous  Mixtures. 

The  conditions  most  favourable  for  obtaining  and 
preserving  the  initial  uniform  movement  of  flames 
are  given  2  as  follows : — 

1.  The  inflammable  mixture  should  be  contained 

in  a  long,  straight,  horizontal  tube  open  at 
one  end  and  closed  at  the  other. 

2.  Ignition  should  be  effected  at  the  open  end 

of  the  tube  by  a  source  of  heat  not  greatly 
exceeding  in  temperature  the  ignition 
temperature  of  the  mixture,  and  not 
productive  of  mechanical  disturbance  of 
the  mixture. 

The  speed  of  the  uniform  movement  then  depends 
upon  four  factors,  to  wit : — 

(a)  The  diameter  of  the  tube. 

(b)  The  material  of  which  the  tube  is  made. 
(This  is  negligible  for  tubes  above  a  certain 

small  diameter.) 

(c)  The  source  of  ignition. 

(d)  The  composition  of  the  gaseous  mixture. 


PROPAGATION  OF  FLAME 


71 


Given  constancy  of  the  first  three  factors,  the  flame 
speed  of  a  given  mixture  may  be  regarded  as  a 
definite  physical  constant  for 
that  mixture. 


Method  of  Experiment. 

The  method  adopted  by 
.Wheeler3  and  his  co-workers 
for  recording  the  speed  of  the 
flames  and  the  general  mode 
of  procedure  may  be  briefly 
summarised  as  follows  :  — 

Tubes  of  different  materials, 
but  frequently  of  glass,  measur- 
ing several  metres  in  length, 
of  diameter  ranging  from  2  mm.4 
to  9.6-5  cms.5  (internal  measure- 
ment) and  open  at  both  ends,  are 
fixed  horizontally  in  a  straight 
line.  The  ends  are  flanged  and 
ground  to  receive  flanged  end- 
pieces  which  are  held  in  position 
by  metal  clips.  Each  end-piece 
is  fitted  with  a  wide-bore  three- 
way  tap.  Glass-covered  platinum 
electrodes  reaching  to  the  centre 
of  the  tube,  leaving  a  spark  gap 
of  3  mm.  are  fused  4  cms.  from 
one  end  (E  in  the  figure).  Screen 


UJ< 


,4._Apparatus  used 
by  Wheeler  and  his  co- 
workers- 


wires  of  metallic  copper,6  0-025  mm.  in  diameter,  are 
threaded   vertically   across   the    tube    through    fine 


72  PROPAGATION  OF  FLAME 

holes  bored  into  the  walls  and  afterwards  sealed  with 
adhesive  material.     In  order  to  avoid  any  irregularity 
in  the  speed  of  the  flame  consequent  upon  the  impetus 
that  might  be  given  by  the  igniting  spark,  the  first 
screen  wire,  W15  is  fixed  some  40  cms.  from  E,  and 
other  screen  wires,  W2,  etc.,  are  fixed  50,  100,  200, 
300,  and  400  cms.  respectively  from  the  first.     It  is 
convenient     to     employ     an     electrical    method    of 
recording  the  time   of   passage  of   flame  along   the 
,    tube.      An   electric   current   is   passed   through   the 
screen  wires  raising  them  to  nearly  red  heat,  so  that 
they  rapidly  melt  as  soon  as  the  flames  touches  them. 
The  current  is  thus  instantly   interrupted   and   the 
time   automatically   recorded    by   the   chronometer. 
This  method  appears  to  yield  very  uniform  results. 
In   beginning   an   experiment,   the   two  end   taps 
are  opened  and  a  rapid  current  of  the  desired  mixture 
of  gases  passed  through.     The  taps  are  now  closed, 
and  the  left  hand  end-piece,  A,  is  removed  by  sliding 
it  downwards  very  gently  in  such  a  manner  as  not 
to  disturb  the  quiescent  gaseous  mixture  in  the  tube. 
A  spark  is  passed  through  E,  whereby  the  mixture  is 
ignited  at  the  now  open  end  of  the  tube. 

In  experimenting  with  gases  yielding  flames  of 
sufficient  actinic  power  to  affect  a  photographic  plate, 
photographic  methods  may  be  employed,  a  revolving 
drum  bearing  the  film. 

In  the  case  of  the  combustion  of  carbon  disulphide 
with  oxygen  or  nitric  oxide  the  flames  are  highly 
actinic,  and  in  consequence  readily  photographed.7 
It  is  possible  also  to  obtain  photographs  in  the  case 
pf  acetylene  and  air,  and  in  this  latter  case  it  is  a 


FLAME  SPEED  OF  METHANE 


73 


particular  advantage,  as  the  uniform  speed  soon 
gives  place  to  the  explosion  wave,  and  may  only 
cover  a  matter  of  20  cms.  in  the  tube,  so  that  the 
percentage  error  in  the  screen  fusion  method  is 
abnormally  great.  By  photography  a  permanent 
record  is  obtained,  which  can  be  examined  at  leisure, 


100 


17-5  CM. 


CM. 


1-35  CM. 


12 


METHANE      PER     CENT 


FIG.  15. — Velocity  of  Flame  in  various  percentages  of  Methane — 
Air  Mixtures  in  Tubes  of  different  Diameters  (Parker). 

the  rate  of  motion  being  readily  determined  by 
markings  made  on  the  photographic  film  with  a 
tuning-fork. 

The  size  of  the  tube  exerts  an  important  influence 
upon  the  rate  of  propagation  of  the  flame.8  In  the 
case  of  mixtures  of  methane  and  air,  for  example, 


74 


PROPAGATION  OF  FLAME 


if  the  diameter  of  the  tube  is  small,  say,  of  the  order 
of  2-5  cms.,  the  velocity  of  the  flame  is  greatly 
retarded  by  the  cooling  effect  of  the  walls.  Where, 
on  the  other  hand,  the  diameter  is  increased  above 
10  cms.  convection  currents  tend  to  unduly  accelerate 
the  velocity  of  the  flame,  which  now  exhibits  a 


5     9  30-5 

DIAMETER     OF  TUBE  (CMS) 


96-5 


FIG. 


16.—  Influence  of  Diameter  of  Tube  upon  Flame  Speed 
(Mason  and  Wheeler). 


.  turbulent  front.  The  flame  appears  to  possess  a 
V  swirling  motion  in  a  direction  nearly  perpendicular 
to  the  direction  of  translation  of  the  flame  front. 
This  is  caused  by  the  rapid  movement  of  the  hot 
gases  from  below  upwards  by  convection.  In  tubes 
of  from  5  to  9  cms.  in  diameter  this  rapid  movement 
is  suppressed,  although  the  shape  of  the  flame  front 


FLAME  SPEEDS  IN  NARROW  TUBES         75 

indicates  the  existence  of  a  definite  movement  of  the 
hottest  gases  towards  the  upper  part  of  the  tube. 

Some  of  the  results  obtained  by  Parker  and  by 
Mason  and  Wheeler  are  given  in  Figs.  15  and  16, 
in  the  latter  case  the  diameter  of  the  tube  being 
plotted  against  the  flame  speed. 

It  will  be  observed  that  in  the  neighbourhood  of 


«-GAS 

FIG.  17. 

9  cms.  there  is  a  slowing  up  in  the  influence  of 
diameter  upon  flame  speed. 

But  this  is  only  temporary,  for  further  increase  in 
the  diameter  shows  that  the  maximum  has  not  been 
reached  even  at  96-5  cms. 

There  is,  on  the  other  hand,  a  lower  limit  to  the 
size  of  the  tube  that  will  allow  a  flame  to  traverse 
it.  If  the  tube  is  diminished  in  size,  a  point  is 
reached  at  which  the  flame  will  traverse  only  a  few 
centimetres,  and  with  further  reduction  the  flame 


76  PROPAGATION  OF  FLAME 

will  not  pass  along  at  all.     The  material  of  the  tube 

X  O 

in  these  cases  plays  an  important  part,  metallic  tubes 
being  better  conductors  of  heat,  cool  the  flame,  and 
prevent  the  passage  of  the  flame  more  readily  than 
glass.  This  may  be  conveniently  demonstrated  by 
connecting  a  slanting  tube,  as  shown  in  Fig.  17,  with 
the  gas  supply,  and  igniting  the  gas  as  it  issues  from 
the  narrow  tube  at  A. 

The  cork  at  B  is  then  removed  and  the  gas 
simultaneously  cut  off.  The  flame,  if  tube  A  is  not 
too  small,  will  pass  down  and  a  slight  explosion 
take  place  in  C.  By  employing  different  tubes  at  • 
A  the  influence  of  size  and  material  can  easily  be 
demonstrated. 

This,  of  course,  is  the  principle  of  the  Davy  Safety 
Lamp,  to  which  reference  has  already  been  made,  for 
wire  gauze  may  be  regarded  as  a  series  of  thin  slices 
of  narrow  tubes  joined  together  transversely. 


Variation  of  Gaseous  Mixture. 

When  ignited  under  precisely  similar  experimental 
conditions,  the  velocity  of  the  uniform  movement  of 
the  flame  rises  as  the  percentage  of  combustible  gas 
increases  above  its  lower  limit  value,  a  maximum 
velocity  being  ultimately  attained,  after  which  further 
increase  of  the  combustible  gas  effects  a  reduction  in 
velocity  until  the  higher  limit  concentration  value 
is  reached,  when  the  flame  ceases  to  pass. 

This  is  well  illustrated  by  the  curves  in  Fig.  18, 
which  depict  the  results  of  several  series  of  experi- 


FLAME  SPEEDS  OF  METHANE 


77 


ments  carried  out  with  varying  mixtures  of  methane, 
oxygen,  and  a  neutral  gas — nitrogen — to  serve  as 
diluent.9 

It  will  be  observed  that  by  increasing  the  proportion 
of  oxygen  not  only  does  the  concentration  of  methane 
at  the  lower  limit  fall  slightly,  but  there  is  a  very 


toor 


AiR 


15-05 


5  .  10 

VOLUME  OF  C       IN  100  VOLUMES   OF  02  AND  N2 


•15 


FIG.  18.  — Flame  Speeds  of  Mixtures  of  Methane  in  Oxygen  and 
Nitrogen  (Mason  and  Wheeler). 


great  increase  in  the  upper  limit  concentration,  pro- 
vided the  proportion  of  oxygen  in  the  oxygen- 
nitrogen  mixture  is  less  than  about  25  per  cent. 
With  oxygen  -  nitrogen  mixtures  containing  more 
than  25  per  cent,  oxygen,  the  lower  limit  of 
methane  rises  slightly.100 


78  PROPAGATION  OF  FLAME 

Further,  the  methane  concentration  yielding  the 
maximum  flame  speed  rises  with  the  proportion  of 
oxygen,  and  yields  a  greatly  enhanced  flame  speed. 
Similar  results  are  obtained  with  mixtures  of  methane 
and  air,  the  latter  being  enriched  with  oxygen.10 


\IOO°/0  0; 


•5  30  60 

VOLUME  OF  CH4  IN  100  VOLUMES  OF  02  AND  N2 

FIG.  19. — Flame  Speeds  of  Methane  in  Oxygen-enriched 
Air  (Pay man). 

These  results  may  be  compared,  in  so  far  as  their 
general  characteristics  are  concerned,  with  those 
given  in  Fig.  18  for  the  speed  of  flames  of  methane 
mixed  with  atmospheres  less  rich  in  oxygen  than 
ordinary  air.11 

The  curve  showing  the  results  for  methane  in  pure 


FLAME  SPEEDS  OF  GASES  79 

oxygen  is  interesting.     The  maximum    flame  speed 
is  5500  cms.  per  second — more  than  fifty  times  the    l^ 
maximum  speed  attained  in  air. 

Further,    the    maximum    speed   in   oxygen  occurs 
with   the   mixture    containing  the  two  gases  in  the   \^~ 
proportions     necessary     for     complete     combustion, 
namely,  one  volume  of  methane  to  two  of  oxygen. 
This  is  interesting  in  view  of  the  observation  that 
when  a  detonation  wave  is  set  up  in  mixtures  of 
methane  and  oxygen,  its  speed  is  greatest  when  the    ^ 
two  gases  are  present  in  equal  proportions,  namely, 
CH4+02. 

In  the  case  of  hydrogen  and  air,  the  measurement 
is  less  easy  inasmuch  as  the  period  of  uniform  move- 
ment does  not,  in  the  majority  of  mixtures,  extend 
over  so  great  a  distance  as  1  metre.  In  contra- 
distinction to  mixtures  of  methane  and  air,  the 
maximum  speed  of  propagation  of  the  flame  does 
not  occur  with  the  mixture  containing  hydrogen  and 
oxygen  in  combining  proportions,  namely,  29-5  per 
cent,  of  hydrogen,  but  with  mixtures  over  the  range 
38  to  45  per  cent,  of  hydrogen.12 

In  Fig.  20  are  shown  diagrammatically  the  relative 
speeds  attained  on  firing  mixtures  of  three  saturated 
hydrocarbons  with  varying  proportions  of  air,13  and 
for  the  sake  of  ready  reference,  the  limit  and 
maximum  flame  speeds  for  various  gases  mixed  with 
air  are  given  in  the  accompanying  table. 

It  will  be  observed  that  the  limit  speeds,  for  both 
the  lower  and  higher  limit  concentration  values  tend 
to  approach  the  same  value  (approximately  20  cms. 
per  second)  for  all  the  gases  concerned. 


80  PROPAGATION  OF  FLAME 

Further,  the  maximum  speeds  of  the  hydrocarbon 
gases  are  practically  the  same,  namely,  about  82  cms. 
per  second,  with  the  sole  exception  of  methane.  The 
value  for  this  last  gas  is  about  67  cms.  per  second. 

It  is  rather  remarkable  to  note  that  in  each 
instance  the  mixture  possessing  the  maximum  flame 
speed  should  contain  more  combustible  gas  than 


100 


" 

U67 
o 


5  10  15 

PER  CENT  COMBUSTIBLE  GAS 
FIG.  20.— Flame  Speeds  of  Hydrocarbon  Gases  (Payman). 

is  required  for  complete  combustion,  save  again  in 
the  case  of  methane.  The  ratio 

oxygen  required  for  complete  combustion 
oxygen  yielding  maximum  flame  speed 

is  approximately  1  to  0-85  by  volume.14  In  the 
case  of  methane,  the  maximum  speed  occurs  with 
the  proportion  required  for  complete  combustion. 


FLAME  SPEEDS  IN  AIR 


81 


Flame  Propagation  in  Air  Mixtures  of  Various 
Combustible  Gases. 


r1  na 

Lower  Limit. 

Maximum 
Flame  Speed. 

Upper  Limit. 

uas. 

Gas. 

Flame 
Speed. 

Gas. 

Flame 
Speed. 

Gas. 

P'lame 
Speed. 

Per  cent. 

(cm.  sec.). 

Per  cent. 

(cm.  sec.). 

Per  cent. 

(cm.  sec.). 

CH4           is 

5'80 

23-3 

9-52 

66-6 

13-35 

19'1 

C2H6 

3-30 

18-1 

6-53 

85-6 

10-60 

19-7 

C3H8           w 

2-37 

20-8 

4-71 

82-1 

7-30 

20-3 

C!H  o       » 

1-95 

20-1 

3-66 

82-6 

6-53 

20-3 

C5H12 

1*61 

20-2 

2-92 

83-0 

5-40 

20-2 

H2    .        ,15 
CH4  +  H2  » 

6-19 
6-03 

10-0 

15-0 

36  -3  12 

14-93 

503-0  12 
135-3 

71-39 
20-80 

50-0 
24-3 

CO   .     15-16 

16-29 

19-5 

44-84 

60-1 

71-19 

19-4 

CH4  +  CO  15 

9-45 

21-9 

15-95 

91-3 

21-55 

19-8 

CO  +  H2  .  15 

9-25 

18-2 

45-92 

315-2 

71-34 

44-4 

C2H2        .« 

3-45 

41-0 

8-9 

282-0 

16'0 

68-0 

Experiments  with  gases  other  than  hydrocarbons, 
such,  for  example,  as  hydrogen  and  carbon  monoxide, 
or  with  mixtures  of  these  with  methane,  do  not 
manifest  the  same  regularities  as  the*  simple  hydro- 
carbon gases.15  This  will  be  evident  from  a  glance 
at  the  curves  given  in  Fig.  21. 

Although  in  most  cases  the  lower  limit  mixtures 
yield  flame  speeds  not  widely  removed  from  20  cms. 
per  second,  in  the  case  of  hydrogen  the  lower  limit 
speed  is  only  10  cms.  Considerable  variation  is 
manifest  in  the  upper  limit  speeds,  that  of  hydrogen 
being  50  cms.,  and  that  of  a  mixture  of  hydrogen 
and  carbon  monoxide  in  equal  proportions  by  volume 
being  44-4  cms.  per  second.  (See  Table.) 

F 


82 


PROPAGATION  OF  FLAME 


The  most  pronounced  speed  variation,  however, 
occurs  with  the  various  maxima.  Thus,  the  maximum 
speed  attained  in  a  mixture  of  hydrogen  and  air 
is  nearly  eight  times  the  maximum  attained  in 
methane  and  air.18 


500 


<J 

LU 

to 


UJ 


50 

VOLUME    OF  COMBUSTIBLE  GAS  PER  100  VOLUMES   OF  AIR 
FIG.  21.— Flame  Speeds  with  H2  and  CO  (Payman). 

B.  Gaseous  Explosions. 

A  gaseous  explosion  may  be  defined  as  a  reaction 

between   two   or   more   gases  which   proceeds   with 

/    rise  of  temperature  and  an  ever-increasing  velocity, 

until   a   maximum  high  velocity  is   attained,  when 

it  becomes  practically  constant. 


GASEOUS  EXPLOSIONS  83 

There  is  thus  no  strict  line  of  demarcation 
between  an  explosion  and  the  uniform  slow  propa- 
gation of  flame  which  was  considered  in  the  previous 
section.  It  is  purely  a  question  of  relative  velocities. 
It  was  not  until  1880  that  the  attention  of 
scientists  generally  was  directed  to  the  fact  that 
practically  nothing  was  known  of  the  velocity  with 
which  explosion  waves  could  travel  in  gases.  In 
that  year  a  coal-gas  explosion  occurred  in  Tottenham 
Court  Road,  and  it  was  concluded  from  the  evidence 


LEAD   COIL 
FIG.  22.— Dixon's  Apparatus. 

that  the  wave  must  have  travelled  with  a  velocity 
of  at  least  100  yards  per  second.  During  the  next 
two  years  two  important  researches  were  published 
on  the  subject,  one  by  Mallard  and  Le  Chatelier19 
in  1881,  and  the  second  by  Berthelot  and  Vieille20 
in  1882.  These  were  followed  by  the  classical 
research  of  Dixon21  in  1893. 

The  apparatus  used  by  Dixon  in  his  experiments 
with  hydrogen  and  oxygen  and  other  inert  gases 
is  shown  diagrammatically  in  Fig.  22,  and  consisted 
of  a  coil  of  lead  piping  some  75  metres  in  length 
which  could  easily  be  placed  in  a  tank  of  water 
and  kept  at  any  desired  uniform  temperature.  Each 


84  PROPAGATION  OF  FLAME 

free  end  was  connected  with  a  horizontal  tube, 
the  one  fitted  with  a  pair  of  platinum  wires  for 
sparking,  and  both  containing  strips  of  silver-foil, 
A  and  B.  On  passage  of  a  spark,  the  explosion 
wave  ruptured  A,  passed  through  the  lead  coil,  and 
on  emerging  at  B  ruptured  the  second  strip.  The 
time-intervals  between  the  ruptures  at  A  and  B  were 
measured  electrically  and  thus  gave  the  velocity  of 
the  explosion  wave.  In  his  experiments  with  chlorine 
and  hydrogen,  Dixon  employed  straight  pipes  of 
wrought  iron,  glass  lined.  The  silver  membranes  were 
coated  with  paraffin  wax  to  prevent  chemical  action. 

As  the   result  of   these   researches   the  following 
facts  have  been  experimentally  established  : — 

(1)  The  velocity  of  explosion  is  independent  of 

the  material  of  which  the  tube  is  made, 
provided  the  diameter  is  above  a  certain 
minimum.  This  is  precisely  what  was  found 
in  the  experimental  study  of  the  uniform 
slow  propagation  of  flame. 

(2)  The  velocity  of  explosion  is  independent  of 

the  diameter  of  the  tube  above  a  certain 
small  limit.  If  only  3  mm.  in  diameter 
no  explosion  will  pass  (Davy),  but  the  rate 
is  the  same  in  a  tube  of  5  mm.  diameter 
as  in  one  of  15  mm. 

(3)  The    explosion   wave    increases    rapidly   in 
V  velocity  from  the  moment  of  its  inception 

until  a  high  maximum  velocity  is  attained, 
after  which  its  rate  of  propagation  is 
uniform. 


PRESSURE  AND  VELOCITY 


85 


This  velocity  is  very  high,  being  several  times 
that  of  sound.  In  the  case  of  hydrogen 
and  oxygen  in  the  proportions  H2  +  0,  the 
velocity  found  by  Berthelot  was  2810  metres, 
and  by  Dixon  2821  metres  per  second  at 
room  temperature — results  showing  a  re- 
markably close  agreement. 

(4)  Increase  of  pressure  was  found  by  Dixon  22 
to  increase  the  velocity  of  explosion,  although 
when  once  a  certain  maximum  pressure  has 
been  reached,  further  increase  does  not 
appear  to  appreciably  alter  the  velocity. 

Influence  of  Pressure  on  the  Explosion   Velocity 
(Dixon), 


Gases. 

Velocity  of  Explosion 
Wave  at  10°  C.  in 

Pressure 

Metres  per  Second. 

H2  +  O 

2627 

200 

2775 

500 

2821 

760 

2856 

1100 

... 

2872 

1500 

(5)  Rise  of  temperature  tends  to  reduce  the 
velocity.  This  is  evident  from  the  following 
data21:— 


Gases. 

Velocity  of  Explosion  Wave  at 
(Metres  per  Second)  : 

10°  C. 

100°  C. 

H2  +  O 
C2H4  +  2O.2 

C2N2  +  02 

2821 
2581 
2728 

2790 
2538 
2711 

86 


PROPAGATION  OF  FLAME 


(6)  The  presence  of  inert  gases  may  lead  either 
to  the  acceleration  or  retardation  of  the 
velocity  of  the  explosion  wave.  If  one  of 
the  combustible  gases  is  in  great  excess, 
it  behaves  like  an  inert  gas  of  similar 
volume  and  density.  This  is  well  illus- 
trated by  the  following  results  in  the  case 
of  oxygen 21 : — 


Gaseous 
Mixture. 

Velocity 
of  Explosion. 

Gaaeous 
Mixture. 

Velocity 
of  Explosion. 

H2  +  0 

2821 

H2  +  0 

2821 

H2  +  O  +  3O 

1927 

2055 

H  +  O  +  5O 

1707 

H2  +  O  +  5N 

1822 

H2  +  O  +  7O 

1281 

H2  +  O  +  7N 

none 

On  the  other  hand,  hydrogen,  on  account  of  its 
low  density,  accelerates  the  velocity  unless 
present  in  very  great  excess. 

(7)  An  explosion  wave  is  characterised  by  in- 

completeness of  combustion.21  Even  electro- 
lytic gas  does  not  wholly  combine  under 
these  conditions.  In  the  case  of  moist 
CO  +  O,  there  is  a  similar  residuum  of 
unburned  gas,  but  no  hydrogen  peroxide 
has  been  detected  as  would  be  expected 
if  Traube's  theory  (see  p.  45)  were 
correct. 

(8)  Water  vapour  exerts  an  important  influence 

upon  the  velocity  of  explosion  of  carbon 
monoxide  and  oxygen.21  Up  to  35°  C.  the 
water  vapour  in  the  saturated  gases  assists 


PRESSURE  IN  EXPLOSIONS 


87 


the  explosion,  but  above  that  temperature 
it  begins  to  act  as  an  inert  gas,  -reducing 
the  velocity.  Thus  : — 


Condition  of  CO+O. 

Per  cent. 
Water  Vapour. 

Velocity  in 
Metres  per  Second. 

Well  dried     . 

nil 

1264 

Saturated  at  10°  C. 

1-2 

1676 

„       35°  C. 

5-6 

1738 

75°  C. 

38'4 

1266 

Pressure  developed  during  an  Explosion. — This 
is  a  problem  of  great  importance  to  engineers,  more 
particularly  in  connection  with  internal  combustion 
engines.23  The  earliest  measurements  were  made 
by  Bunsen 24  who,  by  means  of  a  lever,  determined 
the  weight  that  must  be  placed  upon  a  movable 
lid  resting  on  a  cylinder,  to  prevent  its  being  forced 
off  during  an  explosion  of  gases  within  the  cylinder. 
He  found  as  follows  : — 


CO  +  O 


Pressure  generated. 

1O1  atmospheres 
9-5 


Berthelot  and  Yieille20  obtained  closely  similar 
results. 

The  various  methods  now  in  use  consist  essentially 
in  exploding  gaseous  mixtures  in  metallic  cylinders 
and  automatically  recording  the  pressures  exerted. 
The  pressures  obtained  in  practice  are  never  equal 
to  those  to  be  expected  from  theoretical  considera- 
tions, neither  are  the  temperatures.  There  are  several 


88  PROPAGATION  OF  FLAME 

contributory  causes,  chief  amongst   which   may   be 
cited : —  • 

(a)  Dissociation  of  the  gaseous  products. 

(6)  Incomplete  combustion. 

(c)  Variation  in  specific  heats  of  gases  under 
the  special  conditions,  which  render  the 
theoretical  calculations  uncertain. 


SECTION  IX. 
SURFACE  COMBUSTION. 

IF  a  piece  of  platinum  wire  or  platinised  asbestos 
after  being  warmed  in  a  Bunsen  flame  is,  whilst 
still  warm,  plunged  into  a  stream  of  coal-gas  and 
air  issuing  from  a  Bunsen  burner,  the  whole  begins 
to  glow.  The  coal-gas  is  uniting  with  the  oxygen 
of  the  air  on  the  surface  of  the  platinum,  evolving 
sufficient  heat  to  raise  the  metal  to  incandescence, 
but  usually  without  the  production  of  flame.  If  the 
coal-gas  is  replaced  by  hydrogen,  the  platinum  or 
platinised  asbestos  becomes  sufficiently  hot  to  heat  the 
gas  up  to  its  ignition  temperature,  with  the  result  that 
the  hydrogen  bursts  into  flame  (see  p.  48).  This  was 
first  demonstrated  by  Davy1  in  1818,  who  employed 
a  spiral  of  platinum  wire. 

An  effective  lecture  experiment  illustrating  surface 
combustion  consists  in  inserting  a  short  platinum 
spiral  attached  to  a  glass  rod,  and  gently  warmed, 
into  a  beaker  containing  a  few  c.c.  of  methyl  alcohol. 
The  mouth  of  the  beaker  is  now  loosely  covered  * 
with  a  piece  of  filter-paper,  which  prevents  flames 


90 


SURFACE  COMBUSTION 


from    rising,    and   platinum   glows   brightly   as   the 
alcohol  burns  upon  its  surface  (see  Fig.  23). 


\JUUUUUUUU 


CH3  OH 


FIG.  23. 

This  experiment  is  merely  a  modification  of  a 
well-known  method  of  preparing  formaldehyde,  which 
consists  in  passing  a  mixture  of  air  and  methyl 
alcohol  vapour  over  a  heated  platinum  wire.2  The 
reaction  is  usually  represented  by  the  equation 

CH3OH  =  HCHO  +  H2. 

Formaldehyde 

This  is  followed  by  a  certain  amount  of  further 
dehydrogenation  3 

HCHO  = 


and  subsequent   oxidation,   which  may  be  more  or 
less  complete  according  to  circumstances.     Thus  : 


=  H2O  +  CO2. 


The  reaction,  when  once  started,  is  self-supporting 
and  lamps  have  been  constructed  for  this  purpose.4 

A  pretty  variation  of  this  experiment  for  popular 
demonstration,  consists  in  suspending  by  means  of 


SURFACE  CATALYSIS 


91 


a  copper  wire  a  star-shaped  piece  of  platinum-foil 
from  a  glass  support  in  a  tumbler  or  other  suitable 
glass  vessel,  containing  some  perfumed  alcohol.  The 
platinum  is  first  warmed  and  then  lowered  into  the 
vapour,  and  when  it  has  become  red  hot,  and  flames 
are  escaping  from  the  vessel,  a  metal  lid,  pierced 
with  several  holes  for  ventilation,  placed  on  top. 
This  extinguishes  the  flames,  but  the  platinum  star 
continues  to  glow.  If  the  room  is  darkened  the 
effect  is  very  beautiful. 


FIG.  24. 

Electrolytic  gas,  that  is,  a  mixture  of  two  volumes 
of  hydrogen  with  one  of  oxygen,  may  similarly  be 
fired  by  contact  with  a  platinum  spiral5  warmed 
to  50°  C.  Platinum  black  is  particularly  active  in 
this  respect,6  and  if  a  small  quantity  is  introduced 
into  a  mixture  of  electrolytic  gas  a  violent  explosion 
occurs  as  the  gases  unite  to  form  water. 

This  may  be  demonstrated  in  a  convenient  manner 
by  inserting  a  flask  filled  with  electrolytic  gas  into 
a  heavy  upturned  wooden  box,  the  mouth  of  the 
flask  just  appearing  through  a  small  hole  in  the 
bottom  of  the  box  as  shown  in  Fig.  24.  The  cork 


92  SURFACE  COMBUSTION 

is  now  removed  and  a  trace  of  platinum  black  added. 
The  flask  may  be  blown  to  pieces  by  the  force  of 
the  explosion,  the  fragments  of  glass  falling  harm- 
lessly down. 

Combustions  of  this  kind  are  intimately  associated 
with  and  affected  by  the  surface  conditions  of  the 
catalyser,  and  are  therefore  referred  to  under  the 
general  title  of  surface  combustion. 

The  phenomena  attending  this  type  of  combustion 
have  been  carefully  studied  by  numerous  investigators, 
and  the  correctness  of  the  following  generalisations 
appears  now  to  have  been  established — 

1.  The  property  of  accelerating  gaseous  combustion 
at  temperatures  below  the  ignition-point  is  shared 
by  all  substances  irrespective  of  their  chemical  com- 
position. That  it  is  not  the  peculiar  and  exclusive 
property  of  the  platinum  metals,  nor  indeed  of 
metals  in  general,  was  demonstrated  many  years 
ago.  Thus,  for  example,  in  1887  Fletcher7  showed 
that  Davy's  experiment  may  be  varied  by  allowing 
a  mixture  of  air  and  coal-gas  to  impinge  upon  a 
large  ball  of  iron  wire,  previously  warmed  to  the 
necessary  temperature  to  induce  surface  combustion. 
The  gas  and  oxygen  readily  unite  under  these 
conditions,  and  the  temperature  rises  rapidly,  the 
iron  being  raised  to  incandescence. 

Dulong  and  Thenard8  in  1823  found  that  finely 
divided  silver  causes  the  combustion  of  hydrogen  in 
oxygen  at  150°  C.,  thin  gold  leaves  at  260°  C.,  and 
even  fragments  of  non-metallic  bodies  such  as  charcoal, 
pumice,  porcelain,  quartz,  and  glass  at  temperatures 
below  350°  C.  Curiously  enough,  angular  pieces  of 


COMBUSTION  OF  ELECTROLYTIC  GAS       93 

glass    were   found   to   be  more   efficient   than   glass 
balls  of  equal  superficial  area. 

2.  A  second  interesting  feature  of  surface  com- 
bustion lies  in  the  observation  that  whilst  at  lower 
temperatures  there  exist  very  marked  differences 
in  the  catalysing  powers  of  various  solids,  at  high 
temperatures  not  only  are  all  catalysing  powers 
enhanced,  but  the  aforesaid  chemical  differences  tend 
to  disappear.  At  bright  incandescence  all  solids 
apparently  behave  pretty  much  alike. 


The  Combustion  of  Electrolytic  Gas. 

The  combustion  of  electrolytic  gas  under  varying 
conditions  of  temperature  and  catalysis  has  been 
made  the  subject  of  considerable  investigation. 

Although  in  the  course  of  several  months  a 
mixture  of  hydrogen  and  oxygen  when  moist  and 
exposed  to  daylight  shows  signs  of  chemical  com- 
bination,9 the  action  is  inappreciable  during  the 
course  of  an  ordinary  experiment.  Indeed,  electro- 
lytic gas  has  been  maintained  at  temperatures  as 
high  as  400°  C.  for  a  week  without  showing  any 
appreciable  combination 10  in  the  absence  of  a  catalyst 
other  than  moisture. 

For  a  most  thorough  and  exhaustive  study  of  the 
combustion  of  mixtures  of  hydrogen  and  oxygen  at 
slightly  higher  temperatures  in  contact  with  various 
catalysing  surfaces,  we  are  indebted  to  Bone  and 
Wheeler.11 

Their  apparatus  consisted  essentially  of  a  closed 
system  in  which  the  mixed  gases  could  be  con- 


94  SURFACE  COMBUSTION 

tinuously  circulated  through  a  Jena  hard  glass  com- 
bustion tube  containing  the  surface  under  examination, 
and  maintained  at  any  desired  temperature  in  a  gas 
furnace.  The  gases,  measuring  in  toto  about 
1500  c.c.,  were  kept  in  circulation  by  means  of  a 
mercury  pump,  any  fall  in  pressure  being  readily 
measured.  Arrangements  were  made  also  for 
removing  samples  of  the  gas  whenever  necessary,  in 
order  to  check  the  pressure  records  without  in  any 
way  interfering  with  the  course  of  the  reaction. 
The  electrolytic  gas  employed  in  the  research  was 
prepared  by  the  electrolysis  of  an  aqueous  solution 
of  barium  hydroxide,  thereby  ensuring  its  freedom 
from  ozone,  hydrogen  peroxide,  or  any  hydrocarbon 
impurity.9  The  same  method  was  adopted  also  for 
the  preparation  of  hydrogen  alone,  whilst  if  oxygen 
was  required  it  was  obtained  by  heating  potassium 
permanganate. 

As  catalysts  the  following  types  of  substances  were 
employed : — 

(a)  Refractory  acidic  oxide — porcelain. 
(6)  Refractory  basic  oxide — magnesite. 

(c)  Easily   reducible   oxides — oxides    of    nickel, 

iron,  and  copper. 

(d)  Metals — silver,  gold,  platinum,  and  nickel. 

Porous  Porcelain. — In  the  first  series  of  experi- 
ments the  combustion  tube  was  closely  packed  with 
fragments  of  unglazed  porcelain  which  had  previously 
been  heated  to  1000°  C.  It  was  perfectly  white  in 
colour  and  contained  only  0-05  per  cent,  of  ferric 
oxide.  Electrolytic  gas  was  circulated  through  the 


COMBUSTION  OF  ELECTROLYTIC  GAS       95 


tube  which  was  maintained  at  450°  C.,  and  the 
rate  of  combination  of  the  hydrogen  and  oxygen 
determined.  According  to  the  equation 

2H2  +  02  ==  2H20 

a  reaction  of  the  third  order  might  be  anticipated 
in  accordance  with  the  Law  of  Mass  Action,  and 
indeed  this  is  precisely  what  Bodenstein12  believed 
his  experiments  sufficed  to  establish.  Bone  and 
Wheeler,  however,  have  shown  that  this  is  incorrect. 
The  reaction  is  purely  a  surface  phenomenon,  the  rate 
of  combination  being  directly  proportional  to  the 
pressure  of  the  dry  gas.  This  is  clearly  demon- 
strated by  the  following  table  of  results  obtained 
at  450°  C.,  C0  representing  the  initial  pressure  of 
the  gases,  and  Ct  that  at  time  t. 


Time 
(Hours). 

Pressure  of 
Electrolytic  Gas. 
mm. 

*1  =  llog|o. 

0 

465-6 

36 

163-9 

0-0126 

48 

116-1 

0-0125 

60 

84-6 

0-0123 

72 

60-7 

0-0123 

84 

42-9 

0-0123 

96 

28-6 

0-0126 

108 

20-6 

0-0125 

120 

14-6 

0-0125 

The    reaction   is  thus   seen   to   be   one  of   the   first 
order. 

The  experiments  were  varied  by  mixing  electro- 
lytic gas  with  free  hydrogen  or  oxygen,  so  that 
the  reacting  mixture  contained  an  excess  of  one 


96  SURFACE  COMBUSTION 

of    the    constituents   above    that   required   for    the 
reaction 

2H  +  0    =  2H0. 


It  was  then  found  that  the  rate  of  reaction  of  the 
gases  was  mainly  if  not  entirely  proportional  to  the 
partial  pressure  of  the  hydrogen.  This  result  was 
hardly  to  be  expected  from  a  consideration  of  the 
laws  of  diffusion.  Since  the  rate  of  diffusion  of 
hydrogen  is  four  times  that  of  oxygen  it  follows 
that,  assuming  the  rate  of  combination  at  the  surface 
to  be  very  great,  there  would  always  be  an  excess 
of  hydrogen  at  the  surface  when  ordinary  electro- 
lytic gas  is  employed,  the  rate  of  combination  being 
determined  by  the  rate  of  diffusion  of  the  oxygen. 
Addition  of  oxygen  to  the  system  to  an  amount 
corresponding  to  the  proportion  2H2  +  4O2  should 
therefore  increase  the  rate  of  combustion  to  a 
maximum,  whilst  any  further  addition  either  of 
oxygen  or  hydrogen  would  reduce  it.  The  fact, 
however,  that  this  is  not  the  case  suggests  that 
the  reaction  is  indirect  and  complicated,  and  this  is 
supported  by  the  observation  that  the  catalysing 
power  of  porcelain  is  appreciably  enhanced  by 
exposure  to  hydrogen  prior  to  the  introduction  of 
electrolytic  gas.  This  is  not  due  to  chemical  reduc- 
tion of  the  porcelain,  as  otherwise  the  effect  would 
be  increased  by  prolonged  exposure  to  hydrogen  at 
high  temperatures—  which  it  is  not.  Furthermore, 
the  occluded  hydrogen  can  always  be  completely 
removed  by  pumping  when  the  porcelain  is  at  red 
heat,  although  the  gas  is  obstinately  retained  at  the 
ordinary  temperature. 


COMBUSTION  OF  ELECTROLYTIC  GAS       97 

Previous  exposure  to  oxygen  does  not  appear  to 
stimulate  the  catalytic  activity  of  porcelain  towards 
electrolytic  gas.  The  conclusion  is  therefore  drawn 
— to  quote  the  authors'  own  words — that : 

"  Porous  porcelain  occludes,  or  condenses,  both 
hydrogen  and  oxygen  at  rates  which  depend  to  some 
extent  upon  the  physical  condition  and  past  history 
of  the  surface.  In  general,  however,  whereas  in  the 
case  of  oxygen  the  process  is  extremely  rapid  and 
the  surface  layer  is  soon  saturated,  the  occlusion 
of  hydrogen  is  slower  and  the  limit  of  saturation 
much  higher.  Combination  between  the  occluded 
gases  occurs  at  a  rate  either  comparable  with,  or 
somewhat  faster  than,  the  rate  at  which  the  film  of 
occluded  oxygen  is  renewed,  but  considerably  faster 
than  the  rate  of  occlusion  of  hydrogen." 

Experiments  with  magnesite  at  430°  C.  led  to 
precisely  similar  conclusions.  The  results  of  the 
experiments  with  silver  are  particularly  interesting 
for  two  reasons : 

(a)  The   oxides   of   silver    are    unstable    above 

350°  C.13 

(6)  Analogy  with  sodium  and  copper,14  two 
elements  belonging  to  the  same  vertical 
column  of  the  Periodic  Table,  suggests  the 
possibility  that  silver  may  form  a  hydride 
at  elevated  temperatures. 

It  was  found,  as  with  porcelain,  that  the  rate  of 
combination  of  hydrogen  and  oxygen  in  electrolytic 
gas  in  contact  with  pure  silver  gauze  is  directly 
proportional  to  the  pressure.  Hydrogen  exerts  a 
marked  stimulating  effect. 

G 


98  SURFACE  COMBUSTION 

During  the  course  of  the  experiments  the  surface 
of  the  silver  became  "frosted"  when  heated  to  near 
its  melting-point,15  and  its  catalytic  activity  increased 
some  threefold,  but  was  reduced  again  by  rubbing 
down  the  surface.  The  behaviour  of  the  surface 
suggests  the  formation  of  a  hydride  and  not  mere 
occlusion  or  condensation  such  as  occurs  with 
porcelain. 

Gold  at  250°  C.  showed  acceleration  with  hydrogen, 
but  a  microscopic  examination  of  the  gauze  revealed 
no  sign  of  disintegration  or  of  hydride  formation. 
The  catalytic  action  would  thus  appear  to  be  merely 
an  example  of  superficial  occlusion  or  condensation. 
In  the  presence  of  ferric  oxide  and  of  nickel  oxide 
electrolytic  gas  combines  rapidly  without  producing 
any  change  in  the  catalysing  surface. 

Copper  oxide  behaves  in  an  exceptional  manner. 
Not  only  is  its  action  slow,  but  the  rate  of  formation 
of  water  is  proportional  to  the  partial  pressure  of  the 
oxygen  when  pure  electrolytic  gas  is  not  employed. 

This  is  explained  on  the  assumption  that  a  film  of 
"active"  oxygen  condenses  on  its  surface,  thereby 
protecting  the  catalysing  oxide  from  the  hydrogen 
that  would  otherwise  reduce  it.  At  low  pressures 
this  film  becomes  too  attenuated  to  ensure  complete 
protection,  with  the  result  that  steam  is  formed  by 
the  hydrogen  penetrating  through  to  the  oxide  and 
reducing  it. 

An  interesting  industrial  application  of  surface 
combustion  has  been  devised  by  Bone16  and  consists 
in  heating  a  porous  firebrick  diaphragm,  AB  (Fig.  25), 
by  passing  a  current  of  coal-gas  through  C  into  the 


SELECTIVE  COMBUSTION 


99 


feed  chamber  D  and  igniting  it  on  the  outer  surface 
of  the  diaphragm  as  it  percolates  through.  An 
increasing  supply  of  air,  under  slight  pressure  to 
prevent  "  striking  back,"  is  now  added  to  the  gas 
entering  at  C  until  the  requisite  amount  has  been 
reached.  The  flame  outside  AB  becomes  non- 
luminous,  diminishes  in  size,  and  finally  retreats 
on  to  the  surface  of  the  diaphragm,  the  whole 
surface  layer  of  which  eventually  becomes  red  hot. 


GAS 

+ 
AIR 


FIG.  25. 

There  are  several  interesting  features  attendant 
upon  this  type  of  combustion.  For  example,  the 
heat  is  confined  to  a  thin  layer  of  the  diaphragm, 
usually  less  than  0*25  in.  in  thickness,  the  back  of 
the  diaphragm  remaining  quite  cool.  The  combus- 
tion of  the  gases  is  practically  perfect  and  is 
independent  of  the  external  air  provided  the  gas 
entering  at  C  is  fully  aerated ;  indeed  the  combustion 
will  proceed  with  unimpaired  incandescence  even  in 
an  atmosphere  of  carbon  dioxide. 

Selective  Combustion. — A  useful  field  for  investi- 
gation would  appear  to  lie  in  the  possibility  of 


100  SURFACE  COMBUSTION 

various  catalysing  substances  effecting  selective 
combustions  in  mixtures  of  combustible  gases.  For 
example,  it  has  been  shown 17  that  small  quantities 
of  carbon  monoxide  in  hydrogen  can  be  preferentially 
oxidised  to  carbon  dioxide  by  passing  the  gaseous 
mixture  admixed  with  a  small  quantity  of  oxygen 
over  certain  catalysts.  The  subject  has  been  but 
little  studied. 


BIBLIOGRAPHY   AND    NOTES. 

(The  Abbreviations  used  are  substantially  the  same  as  those 
adopted  by  the  Chemical  Society.} 


SECTION  I. 

1  Using  the  older  data  of  Thomsen.    These  figures  have  been 

revised  by  Mixter,  Amer.  J.  Sci.,  1903,  16,  214. 
la  A  pretty  experiment  illustrating  chlorine  burning  in  hydrogen 
and  hydrogen  in  air  is  described  by  Egerton,  Chem.  News, 
1912,  105,  232. 

2  H.  B.  Baker  (Trans.  Chem.  Soc.,  1902, 81,  400)  has  confirmed 

this  by  showing  that  the  moist  gases  unite  slowly  in  sun- 
light, whilst  the  dried  gases  do  not.  After  four  months 
his  electrolytic  gas  had  contracted  by  4 '35  per  cent. 

3  Meyer  and  Raum,  Ber.,  1895,  28,  2804. 

4  Bone  and  Wheeler,  Phil.  Trans.,  1906,  A,  206,  1. 

5  See  Smithells,  British  Assoc.  Reports,  1907. 


SECTION  II. 

1  For    an    interesting    and    detailed    account  of  the  origin, 

development,  and  final  overthrow  of  the  theory  of 
phlogiston  the  reader  is  referred  to  Freund,  The  Study 
of  Chemical  Composition  (C.  U.  Press,  1904),  Chapter  I. 

2  This   remarkable  disappearance  of  the  phlogiston  during 

calcination  had  not  escaped  the  notice  of  Scheele  (1742- 
1786),  who  evidently  realised  the  anomaly.  (See  Chemical 
Treatise  on  Air  and  Fire,  1777.  Alembic  Club  Reprints, 

No.  8.) 
101 


102  BIBLIOGRAPHY  AND  NOTES 

3  Priestley,  Experiments  and  Observations  on  Different  Kinds 
of  Air,  1774,  ii.,  28.  Alembic  Club  Reprints,  No.  7. 
Scheele  had  already  discovered  oxygen  in  1771,  but  with- 
held publication  for  several  years,  so  that  Priestley's 
discovery  was  the  first  to  be  publicly  announced.  See 
Crell's  Annalen,  1785,  2,  229,  291. 


SECTION  III. 

1  This  theory  was  vigorously  supported  by  Lang  (Zeitsch. 

physikal.  Chem.,  1888,  2,  161),  who  claimed  to  have 
proved  that,  in  the  combustion  of  carbon,  the  formation 
of  dioxide  precedes  that  of  the  monoxide.  His  con- 
clusions, however,  were  disputed  by  Dixon  (Trans.  Chem. 
Soc.,  1899,  75,  630). 

2  C.  J.  Baker,  Trans.  Chem.  Soc.,  1887,  51,  249. 

3  H.  B.  Baker,  Proc.  Roy.  Soc.,  1888,  45,  1 ;  Phil.  Trans.,  1888, 

179,  A,  571. 

4  Rhead   and  Wheeler,  Trans.  Chem.   Soc.,   1912,   101,  846  ; 

1913,  103,  461,  1210. 

6  Above  900°  C.  it  appears  probable  that  reduction  in  pressure 
can  by  itself  effect  the  removal  of  the  oxides. 

6  See   Dewar,  Chem.   News,  1908,  97,   16;  Redgrove,  ibid., 

p.  36;  Aschan,  Chem.  Zeit.,  1909,  33,  561 ;  Dunroth  and 
Kerkovius,  Annalen,  1913,  399,  120.  Contrast,  however, 
Hans  Meyer,  Monatsh.,  1914,  35,  163. 

7  But   see  Rhead    and    Wheeler,  Trans.   Chem.   Soc.,   1913, 

103,  1210. 

SECTION  IV. 

1  Smithells,  J.   Soc.   Ch«n.   Ind.,   1891,   10,  994 ;  Vivian  B. 

Lewes,  ibid.,  1892,  11,  231. 

2  See  Sketches  from  the  Life  of    Sir    Edward    Frankland 

(Spottiswoode,  1902),  pp.  234-56. 

2aSee  Senftleben  and  Benedict  (Kolloid.  Zeitsch.,  1920,  26,  97), 
who  show  that  a  candle  flame  behaves  towards  a  beam  of 
light  in  a  similar  manner  to  a  turbid  medium. 


BIBLIOGRAPHY  AND  NOTES  103 

3  See,  for  example,  Fery,  Compt.  rend.,  1903, 137,  909  ;  Lewes, 

J.  Soc.  Chern.  Ind.,  1892,  11,  231. 

4  Smithells   and    Ingle,  Trans.   Chem.    Soc.,   1892,  61,   204 ; 

Smithells  and  Dent,  ibid.,  1894,  65,  603. 

5  Lewes,  loc.  cit. 

6  On  dismantling  the  experiment,  put  your  finger  at  B  to 

extinguish  the  flame  at  C.    Then  cut  off  the  gas.     The 
gauze  at  A  will  prevent  the  flame  from  "striking  back." 


SECTION  V. 

1  Dixon,  Cantor  Lectures,  1884. 

2  Dalton,  A  New  System  of  Chemical  Philosophy,  1808,  vol.  i. 

3  Kersten,  J.  prakt.  Chem.,  1861,  84,  310. 

4  See  also  Misteli,  J.  Gasbeleuchtung,  1905,  48,  802. 

5  Bone  and  Wheeler,  Trans.  Chem.  Soc.,  1902,  81,  535. 

6  Bone  and  Wheeler,  ibid.,  1903,  83,  1074. 

7  Bone  and  H.  L.  Smith,  ibid.,  1905,  87,  910. 

8  Losanitsch  and    Govitschitsch,    Ber.,    1897,.  30,    136;    de 

Hemptinne,  Bull.  Acad.  Roy.  Belg.,  1897,  34,  269 ;  Solvay 
and  Slosse,  ibid..,  1898,  35,  547. 

9  Armstrong,  Trans.  Chem.  Soc.,  1903,  83,  1088. 

10  Bone  and  Drugman,  ibid.,  1906,  89,  679. 

11  Bone,   British  Assoc.   Reports,   1910,   p.    491 ;    Bone    and 

Stockings,  Trans.  Chem.  Soc.,  1904,  85,  693. 

12  Bone  and    Drugman,    Proc.    Chem.   Soc.,   1904,  20,   127 ; 

Drugman,  Trans.  Chem.  Soc.,  1906,  89,  939. 

13  Bone  and  Stockings,  loc.  cit. 

14  Bone  and  H.  L.  Smith,  Trans.  Chem.  Soc.,  1905,  87,  910. 

15  Bone,   British  Assoc.   Reports,   1910,  p.    491 ;    Bone    and 

Wheeler,  Trans.  Chem.  Soc.,  1904,  85,  1637. 

16  Bone,   British  Assoc.  Reports,   1910,    p.    491 ;    Bone  and 

Andrew,  Trans.  Chem.  Soc.,  1905,  87,  1232. 

17  Dixon,  British  Assoc.  Reports,  1880-,  p.  503;  Chern.  New.-, 

1882,  46,  151 ;  Phil.  Trans.,  1884,  175,  630 ;  Trans.  Chem. 
Soc.,  1886,  49,  95. 

18  Traube,  Ber.,  1882,  15,  666. 

19  Traube's  line  of  argument  appears  weak. 


104  BIBLIOGRAPHY  AND  NOTES 

20  Constam    and    Hansen,    Zeitsch.    Elektrochem.,    1896,    3, 

137,  445;  Bach,  Compt.  rend.,  1897,  124,  2,  951. 

21  Durrant,  Chem.  News,  1896,  73,  228 ;   1897,  75,  43 ;  Proc. 

Chem.  Soc.,  1896,  12,  244. 

22  Smithells  and  Dent,  Trans.  Chem.  Soc.,  1894,  65,  603. 

23  Baker,  Trans.  Chem.  Soc.,  1902,  81,  400. 

24  Bone  and  Haward,  Proc.  Roy.  Soc.,  1921,  A,  100,  67. 

SECTION  VI. 

1  This  illustration  is  borrowed  from  Smithells,  British  Assoc. 

Reports,  1907. 

2  For  example,  a  mixture  of  air    and    vapour    of    carbon 

bisulphide  issuing  into  an  atmosphere  of  nitrogen. 

3  Davy,  Phil.  Trans.,  1816,  p.  7. 

4  Krause  and  V.  Meyer,  Annalen,   1891,  264,  85;  Askenasy 

and  V.  Meyer,  Annalen,  1892,  269,  49 ;  Meyer  and  Raum, 
Ber.,  1895,  28,  2804;  Bone  and  Wheeler,  Trans.  Chem. 
Soc.,  1902,  81,  535 ;  Emich,  Monatsh.,  1900,  21,  1061. 

5  Mallard  and  Le  Chatelier,   Compt.   rend.,   1880,   91,   825 ; 

Meyer  and  Freyer,  Ber.,  1892,  25,  622 ;  Zeitsch.  physikal. 
Chem.,  1893,  11,  28 ;  Meyer  and  Munch,  Ber.,  1893,  26, 
2421 ;  Gautier  and  Helier,  Compt.  rend.,  1896,  122,  566 ; 
Helier,  Ann.  Chim.  Phys.,  1897,  (7),  10,  521 ;  Bodenstein, 
Zeitsch.  physikal.  Chem.,  1899,  29,  665. 

6  See  Dixon  and  Coward,  Trans.  Chem.  Soc.,  1909,  95,  514. 

7  Falk,  J.  Amer.  Chem.  Soc.,  1906,  28,  1517 ;  1907,  29,  1536 ; 

Dixon  and  Crofts,  Trans.  Chem.  Soc.,  1914,  105,  2036. 

8  This  apparatus,  so  beautifully  simple  in  theory,  required  a 

vast  amount  of  manipulative  skill  and  ingenuity  to  bring 
to  perfection  in  practice.  The  student  is  strongly 
recommended  to  study  the  details  as  given  in  the 
original  paper  (Reference  7). 

9  M'David,  Trans.  Chem.  Soc.,  1917,  ill,  1003. 

10  White  and  Price  (ibid.,  1919,  115,  1248)  conclude,  as  the 
result  of  their  experiments,  that  this  method  cannot 
give  the  true  ignition  temperature  and  is  "not  strictly 
trustworthy  even  for  comparative  purposes." 


BIBLIOGRAPHY  AND  NOTES  105 

11  Data  for  coals,   peat,   etc.,   are  given  by  Holm,  Zeitsch. 

angew.  Chem.,  1913,  26,  273. 

12  M'Crea  and  Wilson,  Chem.  News,  1907,  96,  25.    See  resume 

by  Hill,  ibid.,  1907,  95,  169. 

13  This  depends  partly  on  the   composition  of  the  gas  and 

partly  also  on  the  manner  in  which  the  platinised  asbestos 
is  made. 

'SECTION  VII. 

1  Davy,  Collected  Works,  vi.,  p.  24. 

2  Burgess  and  Wheeler,  Trans.  Chem.  Soc.,  1911,  99,  2013. 

3  Burgess  and  Wheeler,  loc.  cit.,  p.  2024. 

4  Burgess  and  Wheeler,  ibid.,  1914,  105,  2591. 

5  Terres  and  Plenz,  J.  Gasbeleuchtung,  1914,  57,  990,  1001, 

1016,  1025.  Confirmed  by  Mason  and  Wheeler,  Trans. 
Chem.  Soc.,  1918,  113,  45,  whose  data  are  given  in  the 
table. 

6  Taffanel,  Compt.  rend.,  1913,  157,  593.     See  also  Burrell 

and  Eobertson,  U.S.  Bureau  of  Mines,  Technical  Paper 
No.  121,  1916  ;  Mason  and  Wheeler,  loc.  cit. 

7  Wheeler  and  Whitaker,  Trans.  Chem.  Soc.,  1917,  ill,  267. 

8  Given  by  Burgess  and  Wheeler,  ibid.,  1911,  99,  2013, 

9  Le  Chatelier,  Legons  sur  le  Carbone,  p.  266. 

10  Clowes,  Proc.  Roy.  Soc.,  1894,  56,  2  ;  1895,  57,  353. 

11  Miiller,  Chem.  Zentr.,  1917,  i.,  991. 

12  Dollwig,  Kolls,  and  Loevenha,  J.  Amer.  Chem.  Soc.,  1917, 

39,  2224. 

SECTION  VIII. 

1  Mallard  and  Le  Chatelier,  Recherches  (Paris,  1883). 

2  Mason  and  Wheeler,  Trans.  Chem.  Soc.,  1917,  111,  1044. 

3  Wheeler,  Trans.  Chem.  Soc.,  1914,  105,  2606. 

4  Payman  and  Wheeler,  ibid.,  1919,  115,  36. 

5  Mason  and  Wheeler,  ibid.,  1917,  ill,  1044. 

6  Parker  and  Rhead  (ibid.,  1914,  105,  2150)  employed  thin 

strips  of  Wood's  alloy,  melting  at  72°  C. 

7  Mallard  and  Le  Chatelier,  Ann.  Mines,  1883,  (8),  4,  312 ; 

Haward  and  Sastry,  Trans.  Chem.  Soc.,  1917,  ill,  841; 
Mason  and  Wheeler,  ibid.,  1919,  115,  578. 


106  BIBLIOGRAPHY  AND  NOTES  . 

8  Parker,  ibid.,  1915,  107,  328 ;   Mason  and  Wheeler,  ibid., 

1917,  111,  1044. 

9  Mason  and  Wheeler,  ibid.,  1917,  ill,  1044;   Burgess  and 

Wheeler,  ibid.,  1914,  105,  2596 ;  Payman,  ibid.,  1919,  115, 
1436. 

10  Payman,  ibid.,  1920,  117,  48. 
10(1  Parker,  ibid.,  1914,  105,  1002. 

11  Numerically    the     results    are    not    strictly    comparable, 

inasmuch  as  the  former  were  obtained  with  tubes  5  cms. 
in  diameter,  whilst  the  latter  were  obtained  with  tubes 
of  2 '5  cms.  diameter. 

12  Haward  and  Otagawa,  Trans.  Chem.  Soc.,  1916,  109,  83. 

13  Payman,  ibid.,  1919,  115,  1446. 

14  Calculated  from  the  data  given  by  Payman  (Reference  13, 

p.  1448). 

15  Payman,  Trans.  Chem.  Soc.,  1919,  115,  1454. 

16  Saturated  with   moisture ;    temperature   12°  C.      Pressure 

75'0  cms. 

17  Mason  and  Wheeler,  Trans.  Chem.  Soc.,  1919,  115,  578. 

18  A  few  experiments  have  recently  been  carried  out  on  the 

vertical  propagation  of  flame.  See  Mason  and  Wheeler, 
ibid.,  1920,  117,  1227. 

19  Mallard  and  Le  Chatelier,  Compt.  rend.,  1881,  93,  148. 

20  Berthelotand  Vieille,  ibid.,  1882,  94,  101 ;  1882,  95,  151,  199  ; 

Berthelot,  ibid.,  1882,  94,  149. 

21  Dixon,  Phil.  Trans.,  1893,  184,  97. 

22  Berthelot  believed  that  alteration  in  pressure    made  no 

appreciable  difference  in  the  velocity. 

23  See  Clerk,  Gas,  Petrol,  and  Oil  Engine  (Longmans,  1909), 

vol.  i. 

24  Bunsen,  Gasometrische  Methoden  (Braunschweig,  1877). 

SECTION  IX. 

1  Davy,  Phil.  Trans.,  1817,  107,  77 ;  Quart.  J.  Sci.,  1818,5, 128. 

2  Hofmann,  Annalen,    1868,   145,   357  ;    Ber.,   1869,   2,   152 ; 

1878,  11,  1686. 

3  Thomas,  J.  Amer.  Chem.  Soc.,  1920,  42,  867.' 


BIBLIOGRAPHY  AND  NOTES  107 

4  Tollens,  Ber.5  1895,  28,  261. 

5  Erman,  Abhandl.  Akad.  Wiss.  Berlin,  1818-19,  p.  368. 

0  Doebereiner,  Schweigger's  J.,  1823,  34,  91;  38,  321;  39, 159; 
42,  60  ;  63,  465. 

7  Fletcher,  J.  Gas  Lighting,  1887, 1,  168. 

8  Dulong  and  Thenard,  Ann.   Chim.  Phys.,   1823,  23,  440; 

1823,  24,  380. 

9  Baker,  Trans.  Chem.  Soc.,  1902,  81,  400. 

10  Bone  and  Wheeler,  ibid.,   1903,  83,  548.     Compare  also 

Meyer  and  Raum,   Ber.,   1895,   28,   2804;    Gautier   and 
Helier,  Compt.  rend.,  1896,  122,  566. 

11  Bone  and  Wheeler,  Phil.  Trans.,  1906,  A,  206,  1. 

12  Bodenstein,  Zeitsch.  physikal.  Chem.,  1899,  29,  665. 

13  Carnelley  and  Walker,  Trans.  Chem.  Soc.,  1888,  53,  79. 

14  Leduc,  Compt.  rend.,  1902,  135,  1332 ;  1903,  136,  1254. 

15  Graham  had  made  a  somewhat  similar    observation    on 

heating  silver  wire  to  redness  and  cooling  in  hydrogen 
(Phil.  Trans.,  1866,  156,  435). 

16  Bone,  J.  Roy.  Soc.  Arts,  1914,  62,  787,  801,  818  •  Coal  and 

its  Scientific  Uses  (Longmans,  1918). 

17  Rideal,  Analyst,  1919,  44,  89;  Trans.   Chem.   Soc.,   1919, 

115,  993. 


INDEX 


ACETYLENE,  43,  51 
Adiabatic  compression,  52 
Ammonia,  61 
Association  theory,  38 

BUNSEN  flame,  26 
temperatures  of,  28 

CALX,  9 

Candle  flame,  19,  68 

Carbon,  combustion  of,  Section 

III. 

Carbon  monoxide,  43,  82,  87 
Charcoal,  13,  68 
Coal-gas  flame,  25 
Coke  furnace,  17 
Combustion,  2 

degraded,  7 

flameless,  7 

preferential,  36 

reciprocal,  34 

selective,  99 

slow,  5 

spontaneous,  6,  48 

surface,  7,  Section  IX. 
Cyanogen,  46 

DAVY  lamp,  .31,  76 
Degraded  combustion,  7 
Detonation,  8 

109 


ELECTROLYTIC  gas,  48,  54,  86, 

93 

Ethane,  42,  51,  80 
Explosion,  8,  82 

velocity,  83 

wave,  69 
Extinctive  atmospheres,  68 

FLAME,  6,  Section  IV. 

propagation  of,  Section  VIII. 
Flameless  combustion,  7 
Flash-point,  55 

table  of,  58 

GASEOUS  explosions,  82 

HEAT  tone,  10 
Heat  of  reaction,  10 
Higher  limits,  62 
Hydrocarbon  gases,  36 
Hydrogen,  46,  54,  82-87 

IGNITION  temperature,  7,  Sec- 
tion VI. 
tables  of,  56,  57,  59 

LIMIT  of  inflammation,  60 
Lower  limits,  62 
Lower  oxygen  limits,  68 


110  INDEX 

Luminosity,  causes  of,  23  KECIPROCAL  combustion,  34 

decrease,  33 

pressure,  31  SELECTIVE  combustion,  99 

temperature,  33  Slow  combustion,  5 

Smithells'  separator,  28 
METHANE,  36-41,  51,    64-67,      Soap  bubble  method,  54 

73-81  Spontaneous  combustion,  6,  48 

Sulphur,  59,  68 

PHLOGISTON,  Section  II.  Surface  combustion,  7,  Section 

Phosphorescence,  7  IX. 

Preferential  combustion,  36 
Pressure  in  explosions,  87  UNIFORM  slow  movement,  70 


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